M represents the MHC-peptide (ligand) and T represents the TCR. In the T cell membrane there are two pools of TCR, one which has access to ligand and another without access to ligand. The TCR is able to "diffuse" between these two pools (A). The TCR is engaged by the ligand and forms a complex (B). The TCR in the complex may either dissociate non-productively or be triggered (C). Triggered TCRs signal until they are internalised and degraded (D).
Up: A Mathematical Model of TCR Signalling