Vaccines

 A vaccine is a material containing weakened or killed pathogens and is used to produce immunity to a disease by stimulating the production of antibodies.

In 1796, a British physician, Edward Jenner, Infected a young boy with cowpox, by injecting pus cells After the boy had recovered from cowpox, Jenner injected the pus cells from a smallpox patient into him. The boy did not get small pox.

image source: Newswise

So it became clear that intentional infection with cowpox protected people from smallpox. This method was named "vaccination" and the substance used to vaccinate was called a "vaccine".

The Mode of Action of Vaccines

Pathogens contain special proteins called "antigens" When pathogens enter the body (blood) of host, these 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.

image source: USC News

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.