Why is penicillin a hapten?

Penicillin is considered a hapten because it is a small molecule that, on its own, does not elicit an immune response. However, when it binds to larger proteins in the body, it can form a complex that the immune system recognizes as foreign, triggering an allergic reaction. This ability to bind to proteins and modify them enables penicillin to act as a hapten, leading to sensitization and potential allergic responses in susceptible individuals.