Antigen

A substance capable of stimulating an immune response by triggering the production of antibodies.

A substance that, when introduced into the body, stimulates the production of protein molecules called antibodies that can bind specifically to the substance.

A substance (often a protein) that induces the formation of an antibody. Antigens are commonly found on the surface of infectious organisms, transfused blood cells, and transplanted organs.

A substance that causes the immune system to make a specific immune response.

Any substance that stimulates the immune system to produce antibodies. Antigens are often foreign substances such as parts of invading bacteria, viruses or parasites.

Any foreign substance, such as a virus, bacterium, or protein, that elicits an immune response by stimulating the production of antibodies. (See Antigenic determinant, antigenic switching.)

(Abbreviation: Ag). A macromolecule (usually a protein foreign to the organism), which elicits an immune response on first exposure to the immune system by stimulating the production of antibodies specific to its various antigenic determinants . During subsequent exposures, the antigen is bound and inactivated by these antibodies. Synonym: immunogen.