Human Health and Diseases - Notes | Class 12 | Part 2: Human Immune System and Immunity



§ It is the system that gives immunity to the body by recognizing, responding and remembering foreign antigens.

§ It plays role in allergic reaction, auto-immune disease and organ transplantation.

§ It includes lymphoid organs, tissues, cells & antibodies.


These are the organs where origin/maturation & proliferation of lymphocytes occur. 2 types: Primary & Secondary.

a. Primary lymphoid organs

The organs where lymphocytes are matured & differentiated to antigen-sensitive lymphocytes. It is 2 types:

1.   Bone marrow: The site of formation of all blood cells including B & T-lymphocytes. 

2.   Thymus: A bilobed organ seen near the heart and beneath the breastbone. It is large during birth but gradually reduces in size and becomes very small size in puberty. Immature T-lymphocytes from bone marrow is migrated to thymus and matured.

b. Secondary lymphoid organs

-    The organs, to which matured lymphocytes migrate from primary lymphoid organs, interact with antigens and then proliferate to become effector cells.

E.g. Spleen, lymph nodes, tonsils, Peyer’s patches, Mucosa- associated lymphoid tissue (MALT) & appendix.

-  Spleen: Bean-shaped organ. Contains lymphocytes and phagocytes. It removes worn-out RBCs & microorganisms from blood. It is a reservoir of erythrocytes in foetus.

-   Lymph nodes: Found in lymphatic system. They trap microorganisms or other antigens. Trapped antigens activate lymphocytes and cause immune response.

-  MALT: Located within the lining of respiratory, digestive & urinogenital tracts. It constitutes 50% of lymphoid tissue.


It is the ability of the immune system to fight the pathogens.

It is 2 types: Innate and Acquired.

1. Innate (inborn) immunity

-    It is the non-specific immunity present at the time of birth.

-    It includes 4 types of Barriers:

a.    Physical barriers: Prevents entry of microbes. E.g. Skin, Mucus coating of the respiratory, gastro-intestinal and urino-genital tracts. Mucus traps microbes.

b.    Physiological barriers: They prevent microbial growth. E.g. gastric HCl, saliva, tear etc.

c.     Cellular barriers: Phagocytes like WBC [Polymorpho-nuclear leukocytes (PMNL) or neutrophils, monocytes and natural killer lymphocytes], macrophages etc.

d.    Cytokine barriers: Virus infected cells secrete a cytokine protein called interferon. It protects non-infected cells from further viral infection.

2. Acquired (adaptive) immunity

·   It is pathogen specific immunity developed during lifetime.

·   It is characterized by memory, i.e. during first encounter of a pathogen, body produces primary response in low intensity. Second encounter of the same pathogen causes a secondary (anamnestic) response in high intensity.

·   Primary and secondary immune responses are carried out with B-lymphocytes (B-cells) and T-lymphocytes (T-cells).

a.    B-lymphocytes: Produce antibodies. These are the proteins to fight the pathogens.

b.    T-lymphocytes: Help B-cells to produce antibodies.

Structure of an antibody molecule

An antibody has 4 polypeptide chains: 2 light chains and 2 heavy chains (H2L2). 

Types of antibodies: IgG, IgA, IgM, IgE & IgD.

Types of Acquired immune response

1.    Humoral immune response/ Antibody mediated immunity (AMI): It is the immune response mediated by antibodies. Antibodies are found in blood plasma. So called as Humoral immune response.

2.     Cell-mediated response / cell-mediated immunity (CMI): It is the immune response mediated by T-lymphocytes (T-cells). The body can differentiate ‘self’ and ‘non-self’ and the CMI causes Graft rejection.

Tissue matching & blood group matching are essential before undertaking any graft/ transplant. After this, the patient should take immuno-suppressants all his life.

Types of Acquired immunity

Acquired immunity is 2 types: Active and passive.

1.    Active immunity: It is the immunity in which antibodies are produced in a host body when the host is exposed to antigens (e.g. living or dead microbes or other proteins).

It is a slow process. It is produced by 2 ways:

a.  Natural Active Immunity: It is developed during natural infection by microbes.

b.  Artificial Active Immunity: It is developed by injecting the microbes deliberately during immunization.

2.    Passive immunity: Here, readymade antibodies are directly given to the body. It is 2 types:

a.  Natural Passive Immunity: E.g.

§ Antibodies (IgG) from mother → Placenta → Foetus

§ Antibodies (IgA) in colostrum → infants

b.  Artificial Passive Immunity: E.g.

§ Anti-tetanus serum (ATS)


This is based on ‘memory’ of the immune system. 2 types:

1. Active Immunization (Vaccination)

§ In this, a preparation of vaccine (antigenic proteins of pathogen or inactivated pathogen) is introduced into the body. It results in the development of antibodies.

§ During actual infection, the antibodies neutralize antigens.

§ The vaccines also generate memory B and T-cells. They recognize the pathogen quickly.

§ E.g. Polio vaccine, Hepatitis B vaccine, DPT vaccine etc.

§ Vaccines are produced using DNA recombinant technology (E.g. Hepatitis B vaccine produced from Yeast).

2. Passive Immunization

§ It is the direct injection of pre-formed antibodies or antitoxin. It is required for quick immune response.

§ E.g. Immunization against Tetanus, snake venom etc.

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