It is the immunity developed by an individual as a result of infection or by specific immunization. It is usually associated with the presence of antibodies having a specific action on the microorganism concerned with a particular infectious disease or on its toxin. The immunity produced is specific for a particular disease.
Active immunity may be acquired in 3 ways
- Following clinical infections - measles, chickenpox
- Sub clinical infections – polio, diphtheria
- Immunization with antigen – may be a killed vaccine or toxoid.
Humoral Immunity: Humoral immunity comes from the B-cells of bone marrow derived lymphocytes, which proliferate and manufacture specific antibodies. The antibodies are located in the immunoglobulin fraction of serum.
Immunoglobulins are of 5 types: IgG, IgM, IgA, IgE and IgD. These antibodies circulate in the body and act directly by neutralizing the organisms or their toxins. The antibodies are specific.
Cellular Immunity: Although antibodies are effective in fighting with infectious diseases, some of the organisms escape the bactericidal action of leukocytes, they can be stimulated by substances secreted by T- lymphocytes or thymus derived lymphocytes. They are effective against some important diseases like tuberculosis, brucellosis.
The T- cells do not secrete antibody but are responsible for recognition of antigen. On contact with antigen, the T-cells initiate the action against the disease causing organism.
Combination of B and T lymphoid cells co-operate with one another to recognize and act on disease causing organism.