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.
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