CBSE Class 12 Biology Chapter 8 Notes

Class 12 Biology Chapter 8 Notes

Class 12 Biology Chapter 7 explains Human Health And Disease. Health is the totality of one’s physical, mental, and social well-being. Health does not just entail being free of sickness or being physically fit. 

The compilation of Chapter 8 Biology Class 12 Notes, which include all the topics and give students a complete explanation, complies with the current CBSE stated criteria and regulations. The Class 12 Biology Chapter 8 Notes can help students study, revise, and reduce their exam load. The academic experts at Extramarks used information from the NCERT textbook and other relevant sources to create these notes.

Students may register on Extramarks to access the Class 12 Biology Notes Chapter 8 and CBSE Class 12 Syllabus. Along with the Syllabus, students may also refer to CBSE Revision Notes, CBSE Sample Papers, CBSE Important Questions, CBSE Extra Questions, and CBSE past years’ question papers.

Key Topics Covered In Class 12 Biology Chapter 8 Notes

Following are the topics included in Biology Chapter 8 Class 12 Notes:

  • Common Diseases in Humans
  • Immunity
  •  AIDS
  • Cancer
  • Drug and Alcohol Abuse


Health is defined as a state of complete physical, mental, and social well-being and not merely the absence of disease or infirmity. There are three dimensions of health, physical, psychological, and social health.

  • Physical health means the perfect functioning of all the parts of the body, and it is determined by certain tests.
  • Mental health is harmony between the individual and the environment. An individual is said to be mentally sound when he is free from stress, anxiety, etc., so the functions are performed in a much better way. It is difficult to assess mental health.
  • Social health is the ability of the individual living in  society to possess the basic requirements of life, like good earnings, good food, clean drinking water, a clean place to live, a happy family, cooperative interaction, and good connection with family and friends, leading to a happy life.The following are necessary to maintain good health. 
  • A balanced diet.
  • Drinking clean water
  • Personal and community hygiene
  • Physical exercises should be regular
  • Disease awareness
  • Vaccination 
  • Waste disposal
  • Control of vectors.


The disease is defined as any physical or functional change from the normal state that causes discomfort or disability or impairs the health of living individuals.

A disease agent is a factor or a substance that produces disease due to its excess, deficiency, or excess.

The types and examples of disease agents are

  • Biological agents: these include viruses, rickettsias, bacteria, fungi, protozoans, helminths, and arthropods. These are called pathogens.
  • Nutrient agents are food components and include carbohydrates and fat, proteins, minerals, vitamins, and water.
  • Endogenous chemical agents are hormones, enzymes, urea, and uric acid.
  • Exogenous chemical agents: these include pollutants like fumes, gases, dust, spores, pollen, etc.
  • Physical agents like heat, cold, humidity, pressure, radiation, electricity, and sound.
  • Mechanical agents like injury, sprain, dislocation, and fracture.
  • Genetic disorders

The term “infection”refers to the entry and multiplication of viruses and organisms in the body of larger organisms. This implies the response of the body to defend itself in some way against the invader. A disease resulting from infection is called an infectious disease.

Classification of disease:

  1. Congenital diseases
  2. Acquired diseases:
  • Contagious or infectious or communicable diseases
  • Non-communicable diseases
  1. Degenerative diseases
  2. Deficiency diseases
  3. Allergies
  4. Cancer Congenital diseases:
  • They have been present since birth.
  • These include anatomical and physiological abnormalities.
  • Causes of congenital diseases are
  1. Single gene mutations like alkaptonuria, phenylketonuria, albinism, sickle cell anaemia, haemophilia, and colour blindness.
  2. Chromosomal abnormalities like Down syndrome and Klinefelter syndrome.
  3. Environmental factors like cleft palate and harelip.

Communicable diseases:

  • These are also known as infectious diseases.
  • These are caused by pathogens like bacteria, viruses, fungi, protozoans, and worms.
  • They spread to other persons.

Non-communicable diseases:

  • These are referred to as non-infectious diseases.
  • They remain confined to the individual.
  • They do not spread from one person to another.
  • These are of four types: organic or degenerative, deficiency diseases, allergy, and cancer.
  • Organic or degenerative diseases are due to the malfunctioning of important organs like the heart disease and epilepsy. 
  • Deficiency diseases are due to the deficiency of nutrients, minerals, vitamins and hormones. Some of the examples are kwashiorkor disease, beriberi, goitre, diabetes, etc.
  • Allergies occur when the person is hypersensitive to foreign substances like hay fever is an example of an allergic disease.
  • Cancer is the uncontrolled growth of the cells of the body.


  • Typhoid:

It is a bacterial disease caused by Salmonella typhi. This is a rod-like bacterium. It is commonly found in the intestine of humans. The disease is common in the age group of 1-15 years.

The disease is transmitted by contaminated food and water. The causative organism enters via the mouth, reaches the intestine, and produces a lesion in the intestinal wall. The incubation period varies from 1-3 weeks.

The symptoms of typhoid are high fever, headache, extreme weakness, abdominal pain, constipation, and inflammation of the ileum and colon. There is an enlargement of the spleen and liver.

Some preventive measures associated with typhoid are proper community sanitation, proper water supply and food screening to check for contamination, and personal cleanliness.

Antibiotics are used to treat the disease.

Widal test: The WIDAL test confirms typhoid. Mary Mallon, nicknamed “Typhoid Mary”, was a professional cook and a carrier of typhoid. Through the food she prepared, she continued to spread typhoid for several years. This is the classic case in medicine.

  • Pneumonia:

Pneumonia is caused by bacteria like Streptococcus pneumonia (commonly called pneumococcus) and Haemophilus influenzae. This is a serious condition of the lungs. The alveoli and bronchioles get collected with fluid, resulting in the lungs not getting enough oxygen to support life.

The mode of transmission of the disease is the sputum of the patient. When pneumococci are inhaled, they get lodged in the bronchioles. As a result, alveolar walls get inflamed, and a protein-rich fluid is secreted. This acts as a culture medium on which the bacteria grow. The incubation period is 1-3 days and is common in older people. Pneumonia shows symptoms of a sudden, shaking chill, fever, pain while breathing, cough, and headache. In severe cases, the lips and fingernails may turn greyish to bluish in colour. The body’s resistance is lowered due to exposure, malnutrition, alcohol, and drug intoxication. Sputum may be bloody or rusty.

  • Common cold:

This is caused by Rhinoviruses. The nose and respiratory passage are infected. The lungs are not infected.

The virus is inhaled through droplets resulting from the coughing or sneezing of the infected person. The symptoms of the disease include nasal congestion, nasal discharge, sore throat, hoarseness, cough, headache, tiredness, etc., and lasti for 3 to 7 days.

  • Malaria:

Malaria is caused by toxins produced by the malarial parasite, Plasmodium, in the human body.

Female Anopheles mosquitoes transmit the malaria parasite from healthy to infected people.The malarial attack is preceded by yawning, tiredness, headache, and muscular pain; the patient may experience chills and shivers during the fever and may have an acute headache, nausea, and a high temperature. The body perspires freely after a few hours, and the body temperature returns to normal. If no medicine is taken, the cycle begins again.The four types of plasmodium species are:

  1. Plasmodium vivax: This causes benign tertian malaria, which attacks every third day after 48 hours. Paroxysms are recurrent episodes of fever.The fever is mild and is seldom fatal.
  2. Plasmodium ovale: These contain the parasitic protozoan that causes tertian malaria in humans.
  3. Plasmodium malariae: It causes quartan malaria, which recurs every fourth day after 72 hours. It is not very common.
  4. Plasmodium falciparum: It is capable of causing three types of malaria, quotidian, which attacks daily, malignant tertian malaria, which recurs every 48 hours (very severe and often fatal) and irregular malaria. The incubation period is 10-14 days for malaria caused by plasmodium vivax.

The life cycle of Plasmodium is completed in two phases and by two hosts. Asexual stages occur in the human host, and the sexual phase occurs in the Anopheles mosquito host. 

The life cycle of Plasmodium:

When Anopheles mosquito bites the infected human being, the malarial parasite enters the mosquito’s body and undergoes further development. These multiply and form sporozoites, reaching the insect’s salivary glands. 

  • Plasmodium enters the human body as sporozoites, the infectious form, through the bite of the infected female Anopheles mosquito.
  • Sporozoites reach the liver via the blood, where they multiply.
  • Then they attack RBC, causing it to burst.
  • Ruptured RBCs release hemozoin. This toxin produces chills and a recurrent fever every 3–4 days.

The methods to control malaria are:

There are two types of measures to control malaria. They are offensive and defensive.

Offensive measure against the mosquitoes include:

  • Draining off or filling up the ditches, ponds, and pools with earth so that there is no place for mosquitoes to breed.
  • If the places are too large, kerosene oil or a solution of DDT should be sprinkled.
  • The drains should be covered or made underground to eliminate the breeding grounds of mosquitoes.
  • Clearance of sluggish drains.
  • Spraying of insecticides like DDT, BHC, etc.
  • House fumigation.
  • Unnecessary vegetation should be removed from and around the houses.

Offensive measures against the parasites include:

  • The malarial parasite can be killed by taking suitable medicines.
  • Quinine was used previously but nowadays, chloroquine and primaquine are also used.
  • Daraprim is also effective.
  • The drugs can be used as prophylactic measures.
  • Dr. Manuel Patarroyo developed a synthetic malaria vaccine in 1987.This proved effective in 60 to 70 percent of cases.

Defensive measures against mosquitoes are

  • In the evening, legs and feet should be covered.
  • Insect repellants should be applied to the exposed parts of the body, especially during the night. This prevents mosquito bites.
  • Sleep under a mosquito net to avoid mosquito bites.
  • Screening of habitation of mosquitoes.
  • Building houses away from swamps and vegetation to avoid mosquitoes.

Amoebic dysentery or Amoebiasis enteritis:

This is caused by Entamoeba histolytica and was first discovered in 1857 by Losch. There are two forms of E. histolytica. 

It is dimorphic, so the larger and more harmful is the Magna form, and the smaller and more harmless is the minute form.

Entamoeba has an irregular body and contains a single nucleus and many food vacuoles in the endoplasmic reticulum. It secretes cytolysin, a proteolytic enzyme that erodes the intestine’s mucous membrane. Dysentery occurs due to bleeding ulcers. The patient passes blood and mucus in his stools. Some amoebiasis symptoms include severe abdominal pain, fever, nausea, exhaustion, and nervousness. Chronic cases are fatal, as the intestinal wall may get punctured. The blood may carry these parasites to the liver, lungs, and brain. 

Some of the important measures to control amoebiasis are

  • Cleanliness and regular hand washing before food handling.
  • Washing fruits and vegetables before use.
  • Food that has been properly cooked kills the cysts.
  • The underground sewage system should destroy human faeces
  • Food protection from insects like cockroaches.
  • Awareness and education.
  • Medications prescribed by the physician to the infected person.


This is caused by a roundworm called Ascaris lumbricoides. This is present freely in the small intestine, which has noorgans for attachment. The mode of nutrition is holozoic, where the roundworm sucks up the host-digested food by sucking through the mouth.

Man became infected after consuming Ascaris eggs with food and water.Children are usually infected by soil ingestion.

The effect produced on the body depends on the degree of infection. A heavy infection may prove fatal because it shares the host’s food and may be responsible for causing anaemia and weakness.

The parasite produces a toxin that causes irritation to the intestinal mucosa and impairs digestion. Coughing, nausea, and colic pain are some of the symptoms.The infection may be associated with poor growth and mental development in children.

The roundworm infection can be confirmed by tests like scratch, dermal, and stool tests.

Filariaisis or Elephantitis:

This is caused by Wuchereria bancrofti. In acute cases, the infection causes fever acute cases, the infection causes fever. In chronic cases, lymphatic vessels are blocked by the worms. This can manifest as swelling of the affected part, which can be the arm, foot, scrotum, or leg. This is followed by the thickening of the subcutaneous tissue and skin.Elephantiasis is the name given to enlarged legs. The disease is not fatal but produces disfigurement, suffering, and disability.

The ways to control the infection are by eradicating the mosquitoes and eliminating breeding places to protect against mosquito bites. Drugs are also available for the treatment of the disease.


Fungi also produce infectious diseases in humans. The fungi that belong to the genera Microsporum, Trichophyton, and Epidermophyton cause ringworms in humans.

The symptoms of the infection are dry, scaly lesions on various parts of the body, like the nails, skin, and scalp. These are also associated with itching. The growth of fungi on the skin is favoured or accelerated by heat and moisture.

 Personal hygiene and infectious diseases:

Personal hygiene is very important to prevent and control infectious diseases. Some of the measures  given in our Class 12 Biology Chapter 8 Notes are

  • Education and awareness about the available vaccine.
  • Isolation of the infected person so that contagious diseases are not transmitted.
  • Vaccination
  • Sanitation
  • Eradication of vectors
  • Sterilisation


Immunity is defined as the ability of an organism to recognise the foreign material or organisms or chemicals that enter the body and mobilise the cells and cell products so that the foreign material may be removed from the body. There are two types of immunity: Innate and acquired immunity.

There are specific defences in the body. The foreign matter that enters the body is known as an antigen or immunogen. This can be a virus, bacteria, etc. The antigen stimulates the immune system to produce protective chemicals to destroy the antigen. These protective chemicals are known as antibodies. These antibodies are protein in nature.

Antigens and antibodies have a fixed reactive site which fits together like the lock and the key.

Innate immunity:

Innate immunity is also known as inborn immunity. Certain animals do not suffer from infectious diseases, as the microbes fail to flourish in these organisms. This type of immunity is passed down from parents and protects from birth to death.That is why it is also known as inborn immunity. Human beings have innate immunity against distemper, which is one of the fatal diseases of dogs.

  • Extramarks Class 12 Biology Chapter 8 Notes explains the four types of innate immunity barriers.They are physical barriers, physiological barriers, cellular barriers, and cytokine barriers.The physical barriers to immunity are skin and mucous membranes like those in the buccopharyngeal cavity, respiratory tract, eyes, and internal tracts. These are the main barriers, so they prevent the entry of microorganisms into the body. Like the mucous that coats the epithelial lining of the respiratory tract, gastrointestinal tract, and urogenital tract, So it traps those microorganisms that are entering the body.
  • Physiological barriers are those that prevent the growth of microorganisms. For example, hydrochloric acid is present in the stomach, saliva is present in the oral environment, and tears are present in the eyes. They all prevent the growth of microorganisms due to the environment provided by them. These are also called chemical barriers, as they play an important role in the defence mechanisms of the body. They are skin secretions and bacteria like lysozyme, saliva, gut secretions like hydrochloric acid and proteolytic enzymes, bile, tears, nasal secretions, cerumen, and salivary bacteria.
  • Cellular barriers like neutrophils, monocytes, macrophages, etc. These cells are present in our bodies. The leukocytes or the WBC like polymorpho-nuclear leukocytes or PMNL – neutrophils, monocytes and the natural killer cells, are present in the blood of our body. The other cells, like macrophages, are present in the tissues of the body, so they can phagocytose the microbes and destroy them. This is the main role of the cellular barrier in our body.
  • Cytokine barriers like interferons Those cells of our body that are infected by viruses secrete interferons. These are the proteins. These protect the non-infected cells so that they are not further affected by the viral infection.

 Acquired immunity:

Acquired immunity is also known as adaptive immunity. This is developed when an animal is exposed to the microbe. This is a very specific type of immunity, and the antibodies mediate it. The antigens are rendered inactive or harmless in this type of immunity. It also prevents further attacks in the future.The main basis of acquired immunity lies in the B and T cells. These cells have a long life. Acquired immunity is of two types, namely, active and passive immunity.

Structure of antibody and role in immunity:

  • There are five types of antibodies that are produced in the body. They are IgA, IgM, IgE, IgG, etc.
  • Because the antibody has four peptide chains, two small light chains, and two large heavy chains, it is known as the H2L2.
  • Antibodies neutralise antigens, which are toxins such as those produced by bacteria and viruses, among other things.The phagocytes dispose of the neutralised antigen-antibody complexes.
  • Antibodies cause particulate antigens to clump together, allowing them to be immobilised and easily disposed of by phagocytes via ingestion.These are termed agglutinins, and the process is known as agglutination.
  • Antibodies combine with antigens to form precipitates that are easily ingested by the phagocytes, and these are known as precipitins; the process is known as precipitation.
  • Opsonization means some of the antibodies coat the surface of the microbes, making them more susceptible to phagocytosis. This is known as opsonins.
  • Complement activation means the formation of the antigen-antibody complex activates the complement proteins, which cause lysis of the bacterium’s cell wall, incite the inflammatory response, and opsonise the antigen and so that phagocytes are attracted to the area of infection.


  • This type of immunity protects the body from viruses, bacteria, and toxins.
  • The B cell is activated when antibodies on a B cell surface bind to the antigen. The daughter cells are formed and specialise into the plasma B cell and the memory B cells.
  • Plasma B cells are the factories of the antibody. These cells are short-lived and are replaced every few days by the bone marrow.
  • The antigen-antibody complex is formed when the antibodies bind to the antigen selectively.
  • The plasma cells do not migrate to the site of infection, and these act through fluid, forming the humoral immune system.
  • Memory B cells are long-lived and serve to quickly dispose of antigen in the event of reinfection by the same virus or bacterium.


  • Cellular immunity is provided by the T cells.
  • Killer T cells migrate to the site of infection and have the surface peptides that are the antigens’ receptors. These cells secrete the perforin protein, which causes puncture of the invader’s cell membrane. These cells also destroy cancer cells.
  • Helper T cells aid in the stimulation of B cells to produce the antibodies; they also stimulate the killer T cells to destroy the body’s own cells.
  • The immune response of both the T and B lymphocytes to antigens is inhibited by the suppressor T cells. These also inhibit the immune system from attacking the body’s own cells.
  • Memory T cells are those cells that produce a rapid attack when the body is infected by the same pathogen again.
  • The killer T cells are the effector cells of the cell-mediated immune system. In contrast, the helper T cells and the suppressor T cells are the regulatory cells of both the antibody-mediated immune system and the cell-mediated immune system.

Cells of the immune system:

  • The primary cells of the immune system are lymphocytes.
  • They arise from the stem cells present in the fetus’ liver and the adult’s bone marrow.
  • Some of them undergo differentiation in the gut-associated bursal lymphoid tissues called tonsils, Peyer patches, and the appendix and are called  B lymphocytes.
  • The other cells are differentiated in the thymus gland and are called thymocytes or  T lymphocytes.
  • The B cells are responsible for the antibody-mediated immune system
  • The T cells are responsible for the cell-mediated immune system.
  • Both of these immune systems need antigens to come into action, but how these two systems respond is very different.


These are the first cells to respond to the infection. The functions of the macrophages are

  • They play an important role in the defence mechanism. Wandering macrophages are also known as histiocytes and are responsible for disposing of the microbes in the connective tissue. The fixed macrophages destroy the microbes in the blood and  lymph.
  • Macrophages fuse, and they form giant cells, which are multinucleated giant cells. These cells capture those  foreign bodies that are too large to be engulfed by the single macrophage.The macrophages act as  scavenger cells and dispose of the dead tissue and  non-cellular foreign matter.
  • These are also responsible for displaying the antigen. These cells carry the antigens of the engulfed microbes on their surfaces and show them to the lymphocytes in the lymphatic nodes,which leads to the activation of the immune system.
  • The macrophages are also responsible for the secretion of endogenous pyrogens, as these macrophages secrete  interleukin 1, also known as endogenous pyrogens, which  has an important role in the inflammatory response and causes fever. These also activate the other phagocytic cells to produce their effect.

Active and Passive immunity:


  • Active immunity is that type of immunity in which the organism’s cells produce antibodies.This occurs when the person gets infected with the disease and is vaccinated. This type of immunity does not provide immediate relief and requires time to form antibodies.
  • Active immunity is somewhat slow and takes time to produce a complete and effective response.
  • Active immunity is induced when the microbes are deliberately injected during the immunisation process. The other way of active immunity may be during the process of natural infection, during which  infectious organisms gain access to the body.


  • Passive immunity means the antibodies are produced by another organism and injected into a person with the potential antigen in the body. Examples include snake venom, rabies, tetanus toxin, and Salmonella infection. The main advantage associated with passive immunity is that it provides immediate relief. The other type of passive immunity is infant immunity.
  • Colostrum is the yellowish fluid secreted by the mother during the early stages of lactation.This contains the antibody IgA in abundance, so it protects the infant.
  •  The infant is passively immune and temporarily protected from the antibodies it receives from the mother’s blood before birth. The nursing mothers also transfer antibodies prepared in their bodies to the infants in their milk.
  • The placenta also transfers some antibodies from the mother to the foetus.

Vaccination and Immunisation:

  • The main basis of vaccination and immunisation is the memory of the immune system.
  • Vaccination introduces pathogen antigenic proteins or inactivated or weakened pathogens.The antibodies are produced against the antigens, and these neutralise the pathogenic agents involved in infection.
  • The vaccines produce B and T cells, which generate memory, and when the same pathogen infects the body, it is identified rapidly and there occurs massive production of antibodies.
  • When a person is infected with the deadly microbe, a quick immune response is required, like in the case of tetanus, so preformed antibodies are injected directly into the body.
  • In cases of snake bites, the injection that is given, is performed with antibodies against the snake venom, this is known as passive immunisation.
  • Vaccines are also performed using the approach of recombinant DNA technology. This is important for the greater availability of vaccines.
  • The production of the Hepatitis B vaccine is done with yeast.

ALLERGY: It is the body’s inappropriate response to a harmless substance that either comes in contact with the body or enters the body from the environment. The substance that causes allergy is known as an allergen. Common allergens are dust, dust mites, pollen, mould spores, feathers, fur, venom, food like eggs, fish, and certain drugs like penicillin.Some of the symptoms of an allergic reaction is inflammation of the mucous membrane, sneezing, gasping, running eyes, irritation of the throat and the trachea, itching of the skin, rashes, etc.

Class 12 Biology Chapter 8 Notes explains some of the conditions of allergy as below:

  • Hay fever: in this allergic condition, pollen is the allergen, and the most common areas affected are the mucous membranes of the eyes and the nose. There is swelling around the eyes and a running nose. An antihistamine pill is given to nullify the histamine.
  • Asthma is a condition in which the reaction occurs farther down in the respiratory tract. The movement of the air through the lungs is impeded. The treatment of asthma is an anti-histamine.
  • Anaphylactic shock is an allergic reaction that occurs in all body tissues. This is a very serious type of allergic reaction. The antigen could be a poison like bee sting, an injectable drug like penicillin, or food consumed orally.The ruptured mast cell releases histamine and basophils, which cause dilation of the arteries throughout the body. There is leakage of a large amount of fluid from the blood into the tissues, and there is a sharp fall in blood pressure. The symptoms of anaphylactic shock cause itchy skin and sudden skin eruptions, vomiting and diarrhoea. There may be swelling of the face, tongue and larynx. The affected individual may lose consciousness, and breathing becomes difficult.  In cases of anaphylactic shock, epinephrine injection can save a person’s life.Antihistamines and corticosteroids are also effective in the treatment of anaphylactic shock.

Autoimmunity occurs when there is a breakage of the body’s self-recognition system. It runs in families, and there is the concept of autoantigens and autoantibodies. These reactions cause various diseases in the body known as autoimmune diseases. If it is RBC and the body’s own RBC is destroyed, then the disease is known as chronic anaemia. If the cells involved are  muscle cells and the body’s muscles are destroyed, it leads to severe muscular weakness. If the autoantigens are the liver cells, then as a result of the destruction of the liver cells, chronic hepatitis results. Some examples of autoimmune diseases are insulin-dependent diabetes, rheumatoid arthritis, Addison’s disease, and ulcerative colitis. The antibodies involved in fighting the streptococcal infection may attack the body’s proteins and  cause damage to the heart valve.


This is a type of genetic disorder with an increased susceptibility to infection. There are two types of immunodeficiency, i.e., primary and secondary immunodeficiency. Primary immunodeficiency is present from birth, and the affected person may lack B and T cells from birth. Due to this, the person is at higher risk of infection. This is a serious congenital disease and is called severe combined immunodeficiency (SCID). In this severe congenital disease,  a defective gene codes for the enzyme adenosine deaminase. As a result of a deficiency of this enzyme, the body becomes defenceless against the disease. The condition is treated with gene therapy.

Secondary immunodeficiency occurs in people whose immune systems have been compromised by prolonged illness, malnutrition, radiation, or drug use.Some of the secondary immunodeficiency examples are AIDS and Hodgkin’s disease.LYMPHOID ORGANS:

  • Lymphoid organs are the sites where the lymphocytes originate.
  • The maturation and proliferation of lymphocytes occur in the lymphoid organs.
  • There are two types: Primary lymphoid organs and secondary lymphoid organs.
  • Primary lymphoid organs include bone marrow and thymus.
  • In primary lymphoid organs, immature lymphocytes are differentiated into antigen-sensitive lymphocytes. And after maturation, these lymphocytes migrate to secondary lymphoid organs.

Bone marrow:

  • Bone marrow is the main lymphoid organ because it is the site where all the cells are produced including lymphocytes.
  • Bone marrow is the site where the development and maturation of B lymphocytes take place.


  • Thymus is a lobed organ which is situated beneath the sternum near the heart.
  • Thymus is larger in size at the time of birth, but with advancing age, the size of the thymus keeps on reducing. This is the site for the maturation of lymphocytes.

Secondary lymphoid organs:

Secondary lymphoid organs are the spleen, lymph nodes, tonsils, Peyer patches of the small intestine  and the appendix. These organs are the site where the antigen and the lymphocytes interact with each other.

Spleen :

The spleen is a large bean-shaped organ. It has white pulp and red pulp. Lymphocytes and phagocytes are present in the spleen. This acts as a reservoir for erythrocytes. The spleen traps the microorganisms that are blood-borne, and because of this, it acts as a filter for the blood.

Lymph nodes:

Lymph nodes are small solid structures and are located at specific sites along the lymphatic system. Those antigens which enter the lymph and the tissue fluid activate the lymphocytes present in the lymph nodes and this leads to the generation of the immune response.


Mucosal-associated lymphoid tissues, or MALT, comprise the lymphoid tissue which is located within the respiratory tract, digestive tract and urogenital tract. They constitute about 50 % of the lymphoid tissue of the human body.

The lymphoid tissue associated with the gut is known as the Gut-associated lymphoid tissue or GALT.

Acquired Immunodeficiency Syndrome (AIDS):

AIDS is a fast-spreading, incurable disease that was first noticed in the USA in the year 1981. The modes of transmission of AIDS are unprotected sexual intercourse with the infected partner, use of contaminated syringes and needles to inject drugs or vaccines, use of contaminated razors for shaving, transfusion of infected blood and blood products, organ transplant, and artificial insemination, from mother to baby. DNA virus is a retrovirus, meaning that the genetic material is RNA and can make a DNA copy of itself with the help of the reverse transcriptase enzyme. The disease occurs when the number of helper T lymphocytes is too low to fight the disease. The incubation period of the virus is between 6 months and 7 years. The average is 28 months. The symptoms of AIDS include destruction of WBC, brain damage, unexplained fever, unexplained loss of appetite, unexplained loss of weight, chronic diarrhoea, cough, night sweats, enlargement of the lymph glands, shortness of breath, and weakness. AIDS can either cause malignant tumours of the connective tissue or the infection of another part of the body. 

The risk groups for AIDS include

  • Homosexuals
  • Drug abusers
  • Blood transfusions.
  • Multiple sex partners.

AIDS virus: Replication and effect

  • The size of the HIV virus is 100 to 140 nm in diameter. The core is cylindrical and is a linear RNA and reverse transcriptase enzyme surrounded by a glycoprotein coat, double lipid membrane, and two protein coats.
  • HIV virus is found in the blood and semen.
  • It is present to a lesser extent in milk, saliva and tears.
  • The virus is a retrovirus which means the genetic material is RNA. A DNA copy can be made by the reverse transcriptase enzyme.
  • The virus binds to the surface receptor on the helper T cell.
  • This is followed by the introduction of RNA and the reverse transcriptase enzyme into the cells.
  • There is multiplication and release of a new virus which destroys the cell.
  • AIDS occurs when there is a gradual decline in the number of Helper T cells.
  • The immune system of the body is too weak to fight against the infection.

The manifestations of the AIDS virus include:

  • Malignant tumour in the connective tissue.
  • Infection related to viral, fungal, protozoal and bacterial.
  • Destruction of WBC
  • Damage to brain
  • Unexplained fever
  • Loss of appetite.
  • Unexplained weight loss
  • Chronic diarrhoea.
  • Cough
  • Night sweats.
  • Lymphadenopathy and enlargement of the lymph glands.
  • Weakness
  • Shortness of breath.

The treatment of AIDS includes

  • Antiviral therapy.
  • Immuno-stimulative therapy.


The various preventive measures for AIDS as explained by Class 12 Biology Chapter 8 Notes are:

  • Disposable needles and syringes should be used.
  • Infected people should not donate blood.
  • There should be a change in sexual habits, like avoiding multiple sex partners, etc.
  • Drug addiction should be controlled.
  • There should be no sharing of personal items like razors, toothbrushes, etc.
  • The blood should be screened for AIDS before  transfusion.
  • Routine screening should be done.


Cancer is defined as the malignant growth of a tissue due to uncontrolled and unlimited cell division. There are three main types of cancer, they are:

  • Sarcomas affect tissues derived from mesoderms, such as the connective tissues, lymphoid tissues, and muscles. Lymphomas are the cancer of the lymphatic tissues, lipomas are formed in the adipose tissue, and osteoma is a cancer of the bones.
  • Carcinomas affect the epithelial and glandular tissues. Examples are breast, lung, pancreatic, and stomach cancer.
  • Leukemias are caused by an excess of WBCs in the bone marrow and lymphatic nodes.

 A property of contact inhibition is shown by the normal cells. This is the property by virtue of which the uncontrolled growth of the cells is inhibited as a result of contact with the other cells. This property is lost by the cancer cells.


A mass of cells is formed when the cancerous cell continues to divide. There are two types of tumours. They are benign and malignant.

Benign tumours are those which do not spread to  other parts of the body, so they are confined to a particular location. The damage caused by benign tumours is little.

Malignant tumours are those that contain neoplastic cells. These cells multiply at an alarming rate, invading and destroying normal tissue in their path.These cells actively divide, and they cause the normal cells to starve due to the competition for vital nutrients. The cells sloughed from this tumour reach various sites of the body via the blood. Wherever these cells are lodged, they start a new tumour there. This is known as metastasis, and it is one of the feared properties of malignant tumours.

The symptoms of the cancer are a non-healing wound, bleeding from an open wound, a lump thatincreases in size, difficulty  swallowing, indigestion, progressive changes in the wart or mole, cough or hoarseness, excessive blood loss in menstruation, sore throat, unexplained weight loss, unexplained low-grade fever, and loss of appetite.

Cancer cells divide in an uncontrolled manner; they do not undergo differentiation; they do not remain confined to parts of the body and tend to spread; they invade neighbouring tissues, and so on.

Causes of cancer:

Class 12 Biology Chapter 8 Notes state the causes of cancer as constant irritation, betel and tobacco chewing, heavy smoking, chemicals, radiation, virus, etc.

Carcinogens in cancer:

  • These are the environmental pollutants that affect the mitotic rate.
  • Some of the carcinogens are soot, coal tar or 3,4 benzopyrene, cigarette smoke, cadmium oxide, aflatoxin, 2-naphthylamine 4-aminobiphenyl, mustard gas, Nickle and chromium compounds, asbestos, diethylstilbestrol and vinyl chloride.
  • These carcinogens cause neoplastic transformation by causing DNA damage.
  • TTobacco smoke contains carcinogens that are common in the development of lung cancer.
  • Oncogenic viruses are viruses that cause cancer, and these viruses have genes, which are called  viral oncogenes.
  • In human normal cells, cellular oncogenes or proto-oncogenes are the genes which have been identified. When these are activated under several conditions,  it causes the oncogenic transformation of the cells.

Chemicals in cancer:

  • These chemicals induce cancer and include nicotine, caffeine, products of coal combustion, oil, pesticides, asbestos, nickel, dyes, etc. 
  • Dye workers have a high rate of bladder cancer.
  • When sex hormones and steroids are secreted in large amounts, they cause cancer.
  • Chimney sweepers tend to develop scrotum cancer.

 Radiation in cancer:

  • X-rays, cosmic rays, and ultraviolet rays are carcinogens.
  • Excessive exposure to UV radiation causes skin cancer.

Class 12 Biology Chapter 8 Notes explain these physical, chemical and biological agents induce cancer and turn the normal cell into a cancerous one. 

Cancer detection:

The cancer is detected by:

  • Biopsy
  • Histopathological examination.
  • Blood test
  • Bone marrow investigations.
  • Radiography
  • Computed tomography or CT.
  • Magnetic Resonance Imaging (MRI).

Treatment of cancer:

The cancer is treated by:

  • Surgery in which the entire cancerous tissue is removed.
  • Exposure to radiation.
  • Chemotherapy in which the anticancer drugs are administered.
  • Immunotherapy.
  • Alpha- interferons are given to activate the immune system and aid in destruction of the tumour.


A drug is a therapeutic substance used in the control, prevention, diagnosis, and treatment of disease. Addiction is defined as a person’s habitual, psychological, and physiological dependence on a substance manifested by stimulation, depression, and distortion of the body’s normal functions.Addiction is a habit beyond voluntary control. The associated person is known as an addict.

The term “abuse” is used when the drug is taken in three aspects: non-prescription, non-medical use, and for pleasure.

Extramarks Class 12 Biology Chapter 8 Notes highlights the features of drug abuse by addicts:

  • The drugs are used without the physician’s advice and are non-prescribed.
  • The drugs are consumed regularly and frequently.
  • Drug taking becomes a habit.
  • Drugs affect the brain and alter consciousness, perception and behaviour.
  • The drugs are used forpleasure. The person takes the drugs secretly for temporary pleasure, which turns into compulsive use.


  • Specific opioid receptors are present in the central nervous system and the gastrointestinal tract. So the opioids bind to these receptors.
  • Heroin, or “smack,” is a white compound that is odourless and bitter. It is a crystalline compound. It is a diacetylmorphine chemically.
  • Heroine or smack is obtained by the acetylation of morphine. It is extracted from the latex of the poppy plant, Papaver somiferum.
  • Heroin is taken by snorting or injecting; it is a depressant and slows down the functions of the body.


  • Cannabinoid receptors are present in the brain and cannabinoids which are a group of chemicals that interact with these receptors.
  • From the inflorescence of the plant Cannabis sativa, natural cannabinoids are obtained.
  • Marijuana is produced from the flower tops, leaves and resin of the cannabis plant. Hashish, charas and ganja are also produced.
  • The mode of addiction is by inhalation or by oral ingestion.
  • These have an effect on the cardiovascular system of the body.


  • This is obtained from the coca plant, Erythroxylum coca.
  • This is native to South America.
  • This interferes with the transport of dopamine, which is a neurotransmitter.
  • It is also known as “coke” or “crack.”
  • It is usually snorted.
  • It is a potent stimulator of the central nervous system.
  • Cocaine produces a sense of euphoria.
  • This increases energy.
  • It could also cause hallucinations when taken in high dosages.
  • There are some other plants that possess the property of hallucination, they are Datura and Atropa belladona.
  • These are also abused by sportspeople.


There are two types of habituating drugs: psychotropic and psychedelic drugs.

Psychotropic drugs:

 These include the following:

  • Tranquilisers like phenothiazines and benzodiazepines reduce tension and anxiety without sedating or inducing sleep. These drugs lessen tension and anxiety and promote a feeling of calmness and soothing.
  • Sedatives and hypnotics, including Barbiturates, benzodiazepines like valium, etc., depress the central nervous system and induce a feeling of calmness, relaxation, and drowsiness. When taken in high concentration, they induce sleep.
  • Opiate narcotics include opium, morphine, codeine, heroin, smack, pethidine, and methadone. These suppress the brain’s activity, provide relief from pain, and are responsible for stimulating the central nervous system.
  • Stimulants include caffeine, cocaine, crack, betel nut, and amphetamines. Their effects include making the person more wakeful, alert and attentive. They induce the feeling of excitement.

Psychedelic drugs:

  • These are hallucinogens.
  • They have an effect on the CNS, altering thoughts, feelings, and perceptions.
  • The illusions may be pleasant or unpleasant.
  • These are also known as vision-producing drugs.
  • They are of two types: Natural and Synthetic hallucinogens.
  • Natural Hallucinogens are Lysergic Acid Diethylamide (LSD), mescaline, psilocybin, hemp products.
  • Synthetic hallucinogens are phencyclidine piperidine (PCP) and methylenedioxy methamphetamine (MDMA).


  • Tobacco is made from the dried leaves of the Nicotiana tabacum and Nicotiana rustica plants.Nicotine is the main substance in tobacco that causes addiction. It acts like a tranquiliser and releases adrenaline due to the stimulation, which leads to high blood pressure.
  • Nicotine is responsible for stimulating nerve impulse passage, causing relaxation of the muscles, increasing blood pressure, retarding foetal growth, and increasing heart rate.
  • Nicotine causes tobacco addiction.
  • The nerves are paralysed by a higher concentration of tobacco.Tobacco smoke contains harmful substances like carbon monoxide, polycyclic aromatic hydrocarbons, and tar.
  • Tobacco is used for smoking, chewing, and snuffing.
  • Tobacco chewing is a common cause of oral cancer.
  • The withdrawal symptoms include irritability, anxiety, craving, sleep problems, headaches, tremors, and lethargy.

Tobacco smoking:

  • Smoking is harmful to your health.
  • It is the most common cause of lung cancer.
  • Tobacco use causes respiratory disease and cardiovascular disease.
  • Respiratory disease includes lung cancer, bronchitis, and emphysema.
  • Coronary artery disease and peripheral vascular disease are both types of cardiac disease.
  • Benzpyrene, which is present in tobacco smoke, is highly carcinogenic.
  • The cigars and pipes irritate the lips and cause lip cancer.
  • Bidi smoking has been linked to cancers of the tongue, tonsils, pharynx, larynx, and esophagus.
  • Carbon monoxide in tobacco smoke reduces the oxygen-carrying capacity of the blood.
  • Pulmonary tuberculosis can be spread from a healthy person to an infected person by smoking bubble gum.
  • Smoking causes damage to the lungs and the immune system.
  • Smoking reduces the immunity of the body, due to which there is an increased risk of infection.
  • Smoking is responsible for reducing the vital capacity of the lungs, due to which the ability for strenuous muscular activities is diminished.


Addiction is the habitual, psychological, and physiological dependence on a substance or practise beyond voluntary control. A person who has an addiction is known as an addict.

Curiosity, peer pressure, pleasure, frustration, and depression all contribute to drug addiction; a desire for more work; a search for a different world; and pain relief.Family history, excitement and adventure, and the availability of drugs.

 Symptoms of drug addiction are excitability, irritability, and violence, an exhausted and drowsy appearance, poor concentration, poor memory, lack of interest in studies, and no interest in social life.

reduced appetite, loss of weight, disturbed sleep, and inattentiveness.

 The withdrawal symptoms of opiates are Nausea, Vomiting, Diarrhoea, Shivering, Perspiration

Twitching, Running nose, Muscular and abdominal cramps, Anxiety, Insomnia, Decreased apetite

Restlessness, Elevated blood pressure, Rapid heart beat, Tremor and epilepsy.


The drugs are prescribed by the physician so that the disease can be treated, prevented, and improved mentally and physically. Once the desired effect is achieved, the drugs are withdrawn. Repeated drug use makes the body depend on them, which is known as drug dependence.

The implications of drug dependence are:

  • The person develops an intense desire for the drug and is taken regularly.
  • When the person stops taking the drug, drug tolerance develops, and withdrawal symptoms begin to surface.
  • The person is unable to decrease the use of the drug.
  • The person is aware of the harm associated with the drug intake but still prefers to consume it.
  • The person starts ignoring basic life activities with a lack of interest.

Extramarks Class 12 Biology Chapter 8 Notes state that Drug dependence is of two types: Psychological and physiological dependence.

In physiologic dependence, the person believes that normalcy can only be achieved through the action of the drug.

In physiological dependence, the person’s state is such that drug use becomes essential to maintain the physiological equilibrium. This is the state in which the nervous system works only in the presence of drugs. This type of physical dependence is known as neuroadaptation.

Some of the factors affecting drug dependence are:

  • The form in which the drug is taken, whether pure or crude.
  • The amount or dosage in which the drug is consumed.
  • The route by which the drug is taken, whether intraoral or intravenous
  • The period over which the drug is consumed is either quick or slow.
  • The susceptibility of the drug abuser.


  • Alcoholism refers to an addiction to alcohol.
  • When alcohol is taken, it is quickly absorbed in the stomach and the upper part of the small intestine.
  • Alcohol reaches all the tissues within minutes.
  • The oxidation of alcohol produces heat. The heat is dissipated as the body does not require it.
  • Alcohol is a depressant, and it dulls the senses.
  • Alcohol’s effects on the nervous system are “anaesthetic”. It interferes with the functions like orientation, alertness, coordination, judgement, memory and mood.
  • Alcohol is responsible for stimulating the secretion of gastric juices from the stomach. This is possible at low dose concentrations of alcohol.
  • At high dose concentrations of alcohol, painful inflammation of the stomach is known as gastritis.
  • The most adversely affected organ as a result of alcohol consumption is the liver. Alcohol is converted to aldehyde in the liver. The liver synthesises fat from alcohol. The formation of glycogen, enzymes and structural proteins is reduced due to the fatty liver.
  • This is how the liver becomes a storehouse of fat, known as fatty liver syndrome.
  • When the fibrous tissue replaces the cells of the liver, then the liver hardens. This type of liver degeneration is known as cirrhosis.
  • When alcohol is taken, the blood vessels quickly dilate to get rid of the heat produced due to oxidation. When the arteries dilate continuously, the walls become rigid and brittle. As a result of this the working of the heart is affected.
  • Alcohol causes severe damage to the kidneys. The kidneys are overworked to eliminate excess water, often taken with alcohol. The skin excretes large amounts of water, and water loss leads to the concentration of nitrogenous waste.
  • A person who is addicted to alcohol may neglect their general health. They have higher chances of developing malnutrition, and the body loses resistance to infections.
  • When alcohol is consumed during pregnancy, there are chances that the foetus is at risk of developing abnormalities and underweight problems. Such children may have facial abnormalities and low IQs.
  • Driving while under the influence of alcohol is strictly prohibited because alcohol impairs the ability to judge distance, reduces coordination and alertness, causes blurred vision, and so on.

 Prevention and control of drug addiction:

  • The habituating drug should be prescribed by a physician only to a genuine person.
  • The drug should be prescribed only for its essential duration.
  • These drugs should not be sold by the pharmacist without a written prescription.
  • The parents should keep a watch on the children. The danger signs should be identified by the parents. Teachers also play an important role here as they may alert the parents of their observations. Also, if the friends discover someone using drugs or alcohol, they must immediately notify the parents so that they can guide the children.On an urgent basis, the cause of the addiction must be ruled out, this will play an important role in initiating the treatment.
  • Social workers should inform family members if they come across a drug abuser. 
  • Undue peer pressure must be avoided, as every child has their own choice, which must be understood and respected by the parents. This is a matter of nurture. Every child has a capacity and capability, be it in sports, education, or other activities, so the child must be encouraged to perform as per his capabilities.
  • If a child is facing any issue related to stress or facing challenges and problems, then he must be educated and counselled to accept that difficulties are part of our lives. Disappointments and failures should be accepted as part of our lives; this should be taught to children.
  • The child must take help from the parents, and they can be guided well by the parents immediately. The help may also come from those who are very close and trustworthy. Asking for a solution to the problem is very helpful for the youth, as they can vent their feelings of guilt and anxiety.
  • It is always important to seek professional and medical health care.

Some of the important steps in de-addiction are pharmacotherapy,  psychosocial therapy,  health restoration,  psychological treatment, and the prevention of relapse.

 Class 12 Biology Chapter 8 Notes: Exercises & Solutions

Students in Class 12 can easily master every chapter of the entire Biology Syllabus by using the educational study tools provided by ExtramarksBiology will be easy to understand once students have access to the most updated syllabus, chapter notes, question-and-answer solutions, etc. Students who sign up for Extramarks get access to the Class 12 Biology Chapter 8 Notes at any time and from any location.

The Class 12 Biology Chapter 8 Notes on Human Health and Disease are created with simple explanations. Based on the CBSE Syllabus, students may find the Notes to be of great use. Students can get exercises and solutions for specific chapters by clicking the links below.

  • Chapter 8: Exercise 8.1 Solutions
  • Chapter 8: Exercise 8.2 Solutions 
  • Chapter 8: Exercise 8.3 Solutions
  • Chapter 8: Exercise 8.4 Solutions
  • Chapter 8: Exercise 8.5 Solutions
  • Chapter 8: Exercise 8.6 Solutions
  • Chapter 8: Exercise 8.7 Solutions
  • Chapter 8: Exercise 8.8 Solutions
  • Chapter 8: Exercise 8.9 Solutions
  • Chapter 8: Exercise 8.10 Solutions
  • Chapter 8: Exercise 8.11 Solutions
  • Chapter 8: Exercise 8.12 Solutions
  • Chapter 8: Exercise 8.13 Solutions
  • Chapter 8: Exercise 8.14 Solutions
  • Chapter 8: Exercise 8.15 Solutions
  • Chapter 8: Exercise 8.16 Solutions
  • Chapter 8: Exercise 8.17 Solutions

Class 12 Biology Chapter 8 Notes study materials and well-crafted notes are available at Extramarks, and they are both accurate and thorough. Students can utilise these study aids to make sense of their questions and comprehend the essential Chapters. These study tools are focused on assisting students in understanding the Chapters included in the CBSE Class 12 textbooks so they can perform well on their Class 12 exams.

Students can access additional sources of instructional content for various classes on the Extramarks website. Click on the relevant links below to learn more.

  • NCERT Solutions
  • CBSE Syllabus
  • Important Formulas
  • CBSE Extra Questions
  • CBSE Revision Notes
  • CBSE Notes
  • Important Questions
  • CBSE Past Year Question Papers
  • CBSE Sample Papers

NCERT Exemplar Class 12 Biology

Biology needs continual revision and research. Students are urged to attempt a lot of questions given the length of the class 12 Biology Syllabus.

The NCERT Exemplar for Class 12 Biology is a great tool for students to use in order to compile questions and solutions from previous years’ exam papers. The Exemplar has a variety of question categories, such as multiple choice, long answers, and short answers.

Our Class 12 Biology Chapter 8 Notes study materials now include comprehensive answer explanations developed by the Extramarks team. The solutions to the solved exemplar problems aid students’ ability to think critically and solve difficulties. Working through these problems will help students fully understand the various topics covered in Chapter 8. In addition to the example solutions, our Class 12 Biology Chapter 8 Notes study materials also include CBSE revision notes, previous years’ exam problems, and other study tools.

Key Features of Class 12 Biology Chapter 8 Notes

The key features of Class 12 Biology Chapter 8 Notes provided by Extramarks are listed below:

  • The CBSE curriculum is applied.
  • The Extramarks faculty members who wrote these notes are highly qualified.
  • It provides a complete understanding of each concept.
  • It focuses on writing concise, in-depth Notes.
  • It provides clear information in an accessible way.
  • It enhances time management skills.
  • It provides a thorough but condensed overview of every subject.

To access all the study materials for the Class 12 Biology Chapter 8 Notes, we suggest students  register on the Extramarks website.

FAQs (Frequently Asked Questions)

1. Which revision notes for Class 12 Biology Chapter 8 are the best?

Every student wants to perform well on their Board Examinations, but to accomplish this, they must concentrate on their studies. They would also require the appropriate instruction to achieve that. You can rely on Extramarks for the best Class 12 Biology Chapter 8 Notes. These are the best revision materials since Extramarks prioritises the calibre of the study materials it offers by drawing on the knowledge of its subject specialists. The revision notes are created in accordance with CBSE recommendations. Students can so easily think that Extramarks Class 12 Biology Chapter 8 Notes are the best.

2. Where can I find the Class 12 Biology Chapter 8 Revision Notes?

For study tools that will give you a precise understanding of Chapter 8 of Class 12 Biology, you can consult Extramarks. You may also locate the Class 12 Biology Chapter 8 Notes that you need to study in order to ace your exams. Having to:

  • View Class 12 Biology Chapter 8 Notes.
  • Visit the option for the relevant chapter.
  • Just click it.

Your device will open the Revision Notes links. You can therefore keep the notes close at hand for future reference.

3. What does Biology Chapter 8 cover for Class 12?

Human Health and Diseases is Chapter 8 of Class 12 Biology. This chapter discusses a variety of health-related subjects in an accessible way. It discusses how diseases can detract from good health and defines health as a person’s mental, physical, and social well-being. It also discusses the general lifestyle and the necessary nourishment. Other than this, this chapter discusses diseases, both contagious and non-contagious ones. Students can easily refer to Extramarks Class 12 Biology Chapter 8 Notes for in-depth knowledge.