Antigen

An antigen is a substance, usually a protein or carbohydrate, that is recognized by the receptors of the adaptive immune system: antibodies, B cell receptors and T cell receptors. Many come from outside the body, such as parts of bacteria and viruses, pollen or toxins. The body's own molecules can act as antigens too, as happens in autoimmune disease and when the immune system targets cancer cells. Not every antigen provokes a response.

Receptors never bind a whole antigen. They recognize a small region of it called an epitope, or antigenic determinant, which on a protein may be only a handful of amino acids, so one large antigen can carry many epitopes. B cells and antibodies bind antigens in their natural shape. T cells, by contrast, only see short fragments that an antigen-presenting cell, such as a dendritic cell or macrophage, has cut up and displayed on MHC molecules.

Vaccines work by showing the immune system a harmless antigen so that it builds memory. Antigens are also used in lab tests such as ELISA, and as markers that identify particular cell types.