Recent observations of T-cell responses following T-cell receptor (TCR) interaction with altered peptide ligands have highlighted the complexity of this signalling system. The h~ dications are that the TCR responds to minor changes in ligand with gradations of T-cell activation and effector functions. Brian Evavold, Joanne Sloan-Lancaster and Paul Allen review these studies and present a model in which partial T-cell activation and TCR antagonism are related events in a continuum of signalling through the TCR.

Traditionally, T-cell activation and the resulting effector functions have been thought to be binary events, being either on or off. However, recent studies have indicated that this view is too simplistic, and that various T-cell effector functions can be uncoupled from each other TM. In the structural analysis of immunogenie peptides, we and others ha,~ e identified peptide analogues which do not induce a T-cell proliferative response, but still bind strongly to defined major histocompatibility complex (MHC) molecules 5-]. The question was then asked, are these analogue peptides capable of delivering any signals to a T cell? Results from six separate groups have now convincingly shown that such peptides have important biological activities (Table 1), and have revealed that the T-cell receptor