State the Principal of Immunisation.

Immunization can be defined as process by which an individual is made immune or resistant to an infectious disease by administration of an external agent (vaccine). The basic principle of immunization is that on administration of the vaccine, the body starts developing antibodies against it such that the person is protected or immunized from the disease. Body remembers the infectious agent and memory cells are formed which can immediately produce antibodies on further exposure to the infectious agent. Hence, the person fights the infection with the help of pre-existing antibodies and is immunized from the disease.