Discuss briefly the principle of immunization.

Immunization - It is the development of resistance against a pathogen. It is carried out through vaccination.

Principle of vaccination. Vaccination introduces a harmless antigen of a pathogen into the body. Our immune system develops some antibodies and memory lymphocytes against the pathogen. When the actual pathogen happens to enter the body, the immune system recognises its antigen through memory cells. The memory cells induce large scale synthesis of cytotoxic and antibody producing lymphocytes to eliminate the pathogen.