Final answer:
Small molecules that need to bind to a carrier to elicit an immune response are known as haptens. They are pivotal in understanding certain allergic reactions and drug sensitivities.
Step-by-step explanation:
Small molecules that do not elicit an immune response unless they are attached to immunogenicity are referred to as haptens. These are essentially free epitopes that are too small to be antigenic by themselves. When a hapten binds to a larger carrier molecule, it forms a conjugate antigen that can then elicit an immune response. The antibodies produced will be specific to the hapten, allowing them to interact with unconjugated free hapten molecules. This is crucial in understanding allergic reactions and drug sensitivities where haptens play a significant role.