How T cells recognize haptens

This report attempts to summarize recent developments in the understanding of T-cell-based immune reactions to haptens, ie, to chemically altered self-proteins. Surprising to some of us, haptens seem to be recognized by exactly the same molecular mechanisms as are “normal” protein antigens, ie, in the form of hapten-modified short peptides tightly associated with major histocompatibility complex (MHC)—encoded MHC antigens. The speciality of haptens, reflected in often unusually strong antigenicity and profound allergenic properties, may relate to the fact that many T cells react to them in an MHC-restricted but carrier-independent fashion. Chemical cell modification thus results in a particularly repetitive array of cross-reactive, immunodominant determinants. The use of synthetic, hapten-modified, MHC-binding peptides now allows detailed molecular analyses of hapten-directed T cell reactivities in vitro as well as in vivo, including hapten-induced contact sensitivity reactions.