Immunity

Immunity (literally 'freedom' or 'exemption') is the traditional term for the host's resistance to injury caused by pathogens and their products — though immune responses are actually much broader, reacting against any foreign antigen, not just infectious ones. The idea of immunity is credited to Edward Jenner in England, who developed the cowpox vaccine that protected against smallpox; immunology is the branch of science that studies the immune system and its responses.

The single most important feature of the immune system is that it can tell the difference between self (the body's own cells) and non-self (foreign molecules or invading cells). Any foreign substance capable of triggering an immune response is called an antigen, and the protective chemicals the immune cells produce in response are called antibodies. Immunity itself is of two broad types — innate (inborn) immunity and acquired (adaptive) immunity — described in the next …