Immunity to a disease is achieved through the presence of antibodies to a specific disease in a person’s system. Antibodies are disease specific and are produced by the body to destroy disease carrying antigens. 

There are two types of immunity: Active and Passive. 

 

Active immunity is acquired through exposure to a diseased organism either naturally (exposed directly to the actual disease) or  vaccine-induced (exposed to a killed or weakened antigen). Both types then stimulate the body to make antibodies, which then attack and eridicate the diseased organisms (antigens). 

 

Passive immunity occurs when an external source of antibodies to a specific disease is given directly to a person. Passive immunity is immediate as opposed to taking several weeks for the body to produce its own antibodies. Passive immunity is quick to attack the antigens but does not offer the long time protection as does active immunity.

 

The body produces 5 classes of antibodies ( immunoglobulins). Of the 5 types, IgG, IgA and IgM are the most prevalent in disease eradication and prevention. Antibodies are produced by white blood cells called  B cell lymphocytes (made in the bone marrow) which activate when exposed to an antigen. This activation causes the B cell lymphocytes to develop into plasma cells which in turn produce the antibodies to destroy the antigen.

 

Antibodies to some diseases develop long term "memory" in cells called T cell lymphocytes and thus are always ready to attack the specific antigen when a person is exposed - even years later. Long term antibody protection from measles an is a good example of this. Some other diseases might require more frequent stimulation of antibody production via revaccination or boosters.

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