When a disease causing microbe enters our body . Our immune system responds against it.And when the same microbe enters our body againĀ 
our immune system responds against it vigorously and eliminates it fastly..
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Immunization is defined as the 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