Class 12 Biology


The ability of our body to fight disease-causing organisms; conferred by the immune system; is called immunity. There are two types of immunity, viz. innate immunity and acquired immunity.

Innate Immunity

The immunity which is present at the time of birth is called innate immunity. It is non-specific type of immunity. Innate immunity is accomplished by providing different types of barriers to the entry of foreign agents into our body. Following are the four types of barriers which constitute innate immunity.

Physical barriers

Skin is the main physical barrier. Mucus coating of the epithelial lining of the respiratory, gastrointestinal and urogenital tracts also act as physical barrier.

Physiological barriers

Physiological barriers prevent microbial growth. Examples: acid in stomach, saliva and tears.

Cellular barriers

Cellular barriers phagocytose and destroy microbes. Examples: certain types of leukocytes, monocytes and lymphocytes in blood, and macrophages in tissues.

Cytokine barriers

Virus-infected cells secrete proteins called interferons which protect non-infected cells from further viral infection.

Acquired Immunity

Acquired immunity is pathogen specific. The body acquires this ability during the lifetime. It is characterized by memory. When our body encounters a pathogen for the first time, it produces a response called primary response. The primary response is of low intensity. Subsequent encounters with the same pathogen elicit secondary or anamnestic response which is of higher intensity.

The primary and secondary immune responses are carried out with the help of two special types of lymphocytes present in our blood. They are; B-lymphocytes and T-lymphocytes.

The B-lymphocytes produce an army of proteins in response to pathogens into our blood to fight with them. These proteins are called antibodies. The T-cells help B cells produce to produce antibodies.

Each antibody molecule has four peptide chains. The two small chains are called light chains, and two longer ones are called heavy chains. Hence, an antibody is represented as H2L2. Antibody mediated immune response is one type of acquired immune response.

The second type of acquired immune response is called cell-mediated immune response or cell-mediated immunity (CMI). The T-lymphocytes mediate CMI. Cell mediated immunity facilitates the rejection of any foreign object. This is the reason tissue matching and blood group matching are required for any graft or transplant.

Active Immunity

When a host produces antibodies in response to an antigen; the immunity is called active immunity. Vaccination involves deliberately injecting microbes into the body. This induces active immunity. Active immunity is slow and takes time to give full effective response.

Passive Immunity

When readymade antibodies are directly given to protect against foreign agents, it is called passive immunity. Mother’s milk during initial days of lactation has abundant antibodies (IgA). It is an example of passive immunity.

Vaccination and Immunisation

The principle of immunization or vaccination is based on the property of ‘memory’ of the immune system. In vaccination, a preparation of antigenic proteins of pathogen or inactivated/weakened pathogen is introduced into the body. The body produces antibodies in response to that. These antibodies shall neutralize the pathogen during actual infection. Vaccines also generate memory. As a result, B and T-cells that recognize the pathogen quickly on subsequent exposure counter the invaders with massive production of antibodies. When a quick immune response is require against some deadly microbe, we need to directly inject the pre-formed antibodies, or antitoxin. Such antitoxins are given in case of tetanus and snake bite. This too is an example of passive immunization.


The exaggerated response of the immune system to certain antigens present in the environment is called allergy. Such antigens are called allergens. Antibodies produced to the allergens are of IgE type. Allergy happens because of release of chemicals like histamine and serotonin from mast cells. Use of drugs like anti-histamine, adrenalin and steroids quickly reduce the symptoms of allergy. Some symptoms of allergic reaction are; sneezing, watery eyes, running nose and difficulty in breathing.

Auto Immunity

Sometimes, the body attacks the self-cells. This happens due to genetic and other unknown reasons. This results in damage to the body, and is called auto-immune disease. Rheumatoid arthritis is an example of auto-immune disease.

Immune System in the Body

The human immune system is composed of lymphoid organs, tissues, cells and soluble molecules like antibodies.

Lymphoid organs

Origin and/or maturation and proliferation of lymphocytes take place in these organs.

Primary Lymphoid Organs

Bone marrow and thymus are the primary lymphoid organs. Immature lymphocytes differentiate into antigen-sensitive lymphocytes in the thymus.

Secondary Lymphoid Organs

Secondary lymphoid organs are; spleen, lymph nodes, tonsils, Peyer’s patches of small intestine and appendix. Interaction of lymphocytes with antigen takes place in secondary lymphoid organs. After that, lymphocytes proliferate to become effector cells.

Bone marrow is the main lymphoid organ where all blood cells (including lymphocytes) are produced. Both bone marrow and thymus provide micro-environments for the development and maturation of T-lymphocytes. Spleen mainly contains lymphocytes and phagocytes. Spleen acts as a filter of the blood by trapping blood-borne microorganisms. Spleen also has a large reservoir of erythrocytes.

Lymph Nodes

Lymph nodes are small solid structures. They are located at different points along the lymphatic system. Lymph nodes trap microorganisms or other antigens which happen to get into the lymph. Antigens trapped in lymph nodes are responsible for activation of lymphocytes present there.

Mucosal Associated Lymphoid Tissue

Lymphoid tissue is also present within the lining of the major tracts. This is called mucosal associated lymphoid tissue (MALT). It constitutes about 50% of the lymphoid tissue in human body.