What is an antigen?

The term antigen refers to something that is recognized by the immune system. This can be something on the surface of a virus, a pollen grain, a toxin produced by a bacteria, or any number of other small molecules. Antigens can come from the environment and be foreign to the body, or they can be something that is normally produced by cells of the body. These latter antigens are called self antigens or autoantigens, and the unwanted response of the immune system against them is the basis of autoimmunity.