Small molecules called haptens trigger immune responses only when linked to carriers. Haptens themselves are too small to be immunogenic but become active when attached to proteins. This property is exploited in vaccine design, allergy studies, and immunoassays to explore antigen-antibody binding specificity and immune reaction mechanisms. https://www.betalifesci.com/blogs/news/antibody-vs-antigen-vs-epitope-vs-hapten-vs-immunogen