The antibiotic penicillin is a small molecule that does not induce antibody formation. However, penicillin binds to serum proteins and forms a complex that in some people induces antibody formation resulting in an allergic reaction. Penicillin is therefore

ANS:D - both an antigen and a hapten

I correct answer is D, because an antigen is a substance which is specifically bound by antibodies or T-lymphocyte antigen receptors and further still which can stimulate the production of or gets recognized by antibodies. Meanwhile, a hapten is a low molecular weight compound which binds with antibodies but does not induce an immune response.

So in this case, a penicillin can specifically bound with antibodies and may produce antibodies in those who react or may not in many people who do not react to penicillins. And in those individuals in whom it does not induce an immune response, its simply a hapten.