The body’s ability to resist or protect itself from the harmful effects of disease producing substance or organisms is called Immunity. Any substance that causes production of antibodies in response of the body metabolism, is known as antigen. Antigen may be bacteria, viruses, or allergens (such as pollen grains) which cause allergy.

Antigens enable the body to protect itself with the help of antibodies produced by lymphocytes (WBC). Immunity could be natural or acquired. Natural immunity is by birth. Acquired immunity develops during lifetime. It develops due to exposure to a disease or by vaccination.

Acquired Immunity

Acquired immunity is of two types:

Active Immunity

Active Immunity develops during exposure to disease causing germs. The body produces antibodies that remain in the blood to prevent further infection by that particular pathogen or disease causing organism. Vaccine containing weakened germs is administered to provide active immunity. For example, DPT vaccine is given for developing immunity against diphtheria, pertusis (whooping cough) and tetanus and BCG vaccine is given for immunity against tuberculosis. 

People also develop immunity against chicken pox, small pox and measles after suffering from these diseases. This form of immunity is usually a life long immunity.

Passive Immunity

This form of immunity is shortlived. It is developed by injecting readymade antibodies (collected from other animals). Anti tetanus serum (ATS vaccine) provides temporary immunity against tetanus.

A vaccine is a sample of an antigen, too small to cause a disease, but enough to produce antibodies. Vaccines have been developed for a number of diseases like polio, mumps, measles, tetanus, diphtheria, and cholera.

Cells Of Immune System

Lymphocytes are cells of the immune system. There are two major types of lymphocytes, T-cells and B-cells, both develop in the Bone Marrow.

A person may lack T-cells or B-cells, or both. Such persons are highly prone to infections.

Immuno Deficiency Disorders

Hereditary, congenital (by birth) or acquired defects in immune response are called Immuno Deficiency Disorders. SCID and AIDS are two common examples of such disorders.

SCID (Severe Combined Immuno-Deficiency Syndrome) is caused due to the absence of both T-cells and B-cells. This defect is present from birth.

AIDS (Acquired Immuno Deficiency Syndrome) causes considerable reduction in T-cells and ultimate destruction of the Immune System. It is caused by HIV (Human Immuno Virus).