Contemporary molecular immunology is now focussed on the cellular aspects of the immune response, particularly the afferent induction pathways. The pathways involve first contacting the antigen, then processing the antigen so that peptide fragments of the native antigen are presented on the cell membrane surfaces of antigen presenting cells in context with major histocompatibility (MHC) molecules. This presentation is made to T helper lymphocytes who express receptors (TCR) for the presented antigen within this MHC context. Representative TCRs have also been sequenced and their structures can be considered analogous to antibody molecules (Fig. 4). Processed antigen molecules fit within the TCR deft in a manner analogous with the way antigen fits in an imunoglobulin deft. Thus even antigen-lymphocyte interactions are governed by the same kinds of intermolecular constraints that antigens and antibodies are subject to.