antigen

An antigen is a foreign molecule that, when introduced into the body, triggers the production of an antibody by the immune system.

The immune system will then kill or neutralize the antigen that is recognized as a foreign and potentially harmful invader. These invaders can be molecules such as pollen or cells such as bacteria.

The term originally came from antibody generator and was a molecule that binds specifically to an antibody, but the term now also refers to any molecule or molecular fragment that can be bound by a major histocompatibility complex (MHC) and presented to a T-cell receptor.

Types of antigens: 

• exogenous × endogenous
• microbial × non-microbial x autoantigens