an·ti·gen 
(ăntĭ-jən) also an·ti·gene (-jēn′)
(ăntĭ-jən) also an·ti·gene (-jēn′)n.
A molecule that is capable of binding to an antibody or to an antigen receptor on a T cell, especially one that induces an immune response. An antigen is usually a foreign substance, such as a toxin or a component of a virus, bacterium, or parasite.
an′ti·genic (-jĕnĭk) adj.
an′ti·geni·cal·ly adv.
an′ti·ge·nici·ty (-jə-nĭsĭ-tē) n.