∞ generated and posted on 2016.03.19 ∞
Otherwise non-immunogenic small molecule that, upon binding to a larger molecule, can induce a humoral immune response.
Penicillin and urushiol are the most familiar of haptens, which in both cases bind to existing body proteins, resulting in an immune response against not just the combined molecules but, once antibodies have been formed, against isolated, individual hapten molecules as well.
In general, for a substance to be antigenic it must be relatively large, and in a way haptens as immunogenic small molecules are the exception that proves the rule (the exception here is the apparent immunogenicity of a small molecule, but it turns out that it is not the small molecule that is immunogenic after all, but instead a large molecule that has the hapten as one component). That is, haptens are immunogenic only when combined into a sufficiently large molecule but not antigenic on their own.
Urushiol, by the way, is the rash-producing molecule associated with poison ivy and poison oak, etc.