Antigen
molecule capable of inducing an immune response (to produce an antibody) in the host organism / From Wikipedia, the free encyclopedia
An antigen is a molecule on the outside of a pathogen (a "germ"). It causes the production of antibodies which stick to the invading virus or bacterium. This is an immune response.[1]
Antigens are usually a protein on the outside of a bacterium or virus. It gets recognized by the adaptive immune system as foreign, and this stimulates the production of antibodies. The antibodies combine with the antigens and make the invading virus or bacterium ineffective. Antigens stimulate the production of antibodies: they do not produce them directly. Vaccines for the seasonal flu virus is a common example.
The body system is normally tolerant of its own molecules, which don't trigger an attack. Autoimmune diseases are caused when this safeguard fails.
The first time that a new antigen comes into contact with the body the response of the immune system will be a complete immune response. During this first response, the antigen will cause antibodies to be made.[2]
The next time the same antigen contacts the body, a full-scale immune response is not needed as the body already has a specific antibody available instantly for that antigen.
Vaccinations usually contain dead bacteria or antigen so the antibodies can recognise it later and make antibodies faster.