Immunity and Immune system


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 and soluble molecules like antibodies.

Lymphoid organs

These are the organs where origin, maturation & proliferation of lymphocytes occur. 2 types.

a. Primary lymphoid organs

Here, immature lymphocytes differentiate into antigen-sensitive lymphocytes. E.g. Bone marrow and thymus.
Bone marrow is the site of formation of blood cells.
Thymus is large during birth but gradually reduces in size and becomes very small size in puberty.

b. Secondary lymphoid organs

Matured lymphocytes migrate to these organs, interact with the antigens and then proliferate to become effector cells.

E.g. Spleen, lymph nodes, tonsils, Peyer’s patches, 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.

Mucosal associated lymphoid tissue (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 disease-causing organisms. It is 2 types: Innate and Acquired.

1. Innate immunity

- It is the non-specific type of defence that is present at the time of birth.

- It provides barriers to the entry of foreign agents into our body.

- Barriers are 4 types:
  • Physical barriers: E.g. Skin (Prevent entry of foreign bodies), Mucous coating of the respiratory, gastro-intestinal and urino-genital tracts to trap microbes.
  • Physiological barriers: E.g. HCl in stomach, saliva, tear etc.
  • Cellular barriers: Phagocytes like WBC [e.g. neutrophils or Polymorphonuclear leukocytes (PMNL), monocytes and natural killer lymphocytes], macrophages etc.
  • Cytokine barriers: Virus infected cells secrete proteins called interferon which protect non-infected cells from further viral infection.
2. Acquired immunity

Pathogen specific immunity.

Characterized by memory, i.e. when our body encounter a pathogen for the first time, produces primary response which is of low intensity. Subsequent encounter with the same pathogen elicits a highly intensified secondary (anamnestic) response.

The primary and secondary immune responses are carried out with B-lymphocytes and T-lymphocytes.
  • B-lymphocytes (B-cells): Produce antibodies.
  • T-lymphocytes: Help B-cells to produce antibodies.
Structure of an antibody molecule

Each antibody has 4 polypeptide chains, 2 small light chains and 2 larger heavy chains (H2L2).

Different types of antibodies: IgG, IgA, IgM, IgE & IgD.

Acquired immune response
2 types.

1. Humoral or Antibody mediated response/ Antibody mediated immunity (AMI): Antibodies are found in blood plasma. So called as Humoral immune response.

2. Cell-mediated response / cell-mediated immunity (CMI):

- T-lymphocytes (T-cells) mediate CMI.

- CMI causes Graft rejection.

- The body is able to differentiate ‘self’ and ‘non-self’.

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

Active Immunity and Passive immunity

1. Active immunity: 

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: During natural infection by microbes.

b.Artificial Active Immunity: Injecting the microbes deliberately during immunization.

2. Passive immunity: 

Here, readymade antibodies are directly given to protect 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)

A preparation of vaccine (antigenic proteins of pathogen or inactivated pathogen) is introduced into body.

The antibodies produced in the body against the antigens neutralize the pathogenic agents during actual infection.

The vaccines also generate memory B and T-cells that 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 when quick immune response is required
E.g. Immunization against Tetanus, snake venom etc.


It is the exaggerated response of the immune system to certain antigens present in the environment.

Allergens: Substances causing immune response. E.g. mites in dust, pollens, animal dander, fur etc.
Antibodies produced against the allergens are of IgE type.

Symptoms: Sneezing, watery eyes, running nose, skin rashes, difficulty in breathing etc.

Allergy is due to the release of chemicals like histamine and serotonin from the mast cells.

To determine the cause of allergy, the patient is exposed to or injected with very small doses of possible allergens, and the reactions studied.

Drugs like anti-histamine, adrenaline and steroids quickly reduce the symptoms of allergy.

Modern-day life style results lowering of immunity and more sensitivity to allergens.

Asthma: Respiratory disease due to allergy.


Sometimes, due to genetic and other unknown reasons, body attacks self cells. This results in auto-immune disease. E.g. Rheumatoid arthritis.


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