What is the term for a molecule that can elicit an immune response only when attached to a carrier?

The term that refers to a molecule capable of eliciting an immune response only when it is attached to a carrier is known as a hapten. Haptens are small molecules that, by themselves, are not immunogenic; they cannot provoke an immune response alone. However, when they bind to larger carrier proteins (often referred to as carrier proteins), the complex can be recognized by the immune system. This association allows the immune system to generate antibodies against the hapten.

In contrast, an immunogen is a substance that can provoke an immune response by itself, which is different from a hapten. Adjuvants are substances that enhance the body's immune response to an antigen, but they do not become part of the immune response themselves, and thus are not applicable to the definition of a hapten. The role of carrier proteins is significant in this context, but they serve to enable the hapten's recognition rather than being the molecule that elicits the immune response.