Haptene

Definition
noun, plural: haptenes
A substance that induces immune response when bound to a carrier protein or to large antigenic molecule.
Supplement
Haptene is a type of antigen that elicits production of antibodies only when combined with another antigenic molecule, such as immunogen. It can however react with previously existing antibodies.
A well known example is urushiol, which is the toxin found in poison ivy.
Word origin: German : Greek haptein, to fasten + German -en, n. suff. (from Greek -ēnē, -ene).
Variant: hapten

Also called: partial antigen or incomplete antigen
See also: immunogen, antigen.