A hapten is defined as what?

• A. Whole antigen
• B. Immunogen
• C. Epitope
• D. Partial antigen

Answer: (D)

A hapten is an incomplete antigen: it’s a small molecule that cannot by itself provoke an immune response, though it can be recognized by antibodies. It becomes immunogenic only when it is covalently linked to a larger carrier protein, which provides the necessary T‑cell help to generate antibodies against the hapten-carrier complex. That’s why the correct description is a partial antigen. The other terms don’t fit: a whole antigen can trigger immunity on its own; an immunogen is something that can provoke an immune response by itself; and an epitope is the specific site on an antigen that antibodies bind, not the hapten’s overall defining concept.