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

• A. an antigen
• B. an immunogen
• C. both an antigen and a hapten
• D. a hapten

Answer: (C)