The mechanism through which the body protects itself from subsequent INFECTION with the same PATHOGEN. Immunization may occur as a natural consequence of infection or through vaccination (also called inoculation). The IMMUNE SYSTEM creates unique antibodies, specialized proteins that attach to B-cell lymphocytes that circulate in the BLOOD and the LYMPH, in response to antigens present on the cell surfaces of the pathogen.
The antibodies then respond to the presence of the ANTIGEN within the body, activating a rapid IMMUNE RESPONSE to contain and neutralize the pathogen before it establishes sufficiently to cause illness. A VACCINE contains a nonpathogenic preparation of an antigen to stimulate the same immune response without causing illness. Natural immunization is usually lifelong. Immunization acquired through vaccination may require a series of vaccinations or periodic booster vaccines to maintain adequate protection against infection.