Ligands for the T-cell receptor: hard times for avidity models.

T-cell activation occurs when the T-cell receptor (TCR) binds to a self major histocompatibility complex (MHC) molecule carrying a specific peptide. Small changes in peptide structure can alter or inhibit this response. Although this phenomenon is normally attributed to a lowering of the avidity of the ligand for its receptor, recent data suggest that conformational changes in the receptor itself may be playing a critical role, as discussed here by Charles Janeway.