Answer: hapten

AnswerCrossword Clue
haptena substance similar to an antigen, also HAPTENE
haptena small separable part of an antigen that reacts specifically with an antibody but is incapable of stimulating antibody production except in combination with a carrier protein molecule
haptensubstance having a single antigenic determinant that can react with a previously existing antibody but cannot stimulate more antibody production unless combined with other molecules; partial antigen