Pathogens contain special protein called “antigens”. When pathogens enter body of host, there proteins stimulate the immune response in host i.e. synthesis of “antibodies”. Antibodies bind to pathogens and destroy them. In addition, “memory cells” are produced, which remain in blood and provide protection against future infections with the same pathogen.
When a vaccine i.e. weakened or dead pathogen is introduced into bloodstream, the white blood cells are stimulated, B-lymphocytes recognize the weakened or dead pathogens as enemies and start producing antibodies’ against them. These antibodies remain in blood and provide protection against pathogens. If real pathogens enter blood, the already present antibodies kill them.
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