Fig. 2 When serological studies are extended quantitatively, affinity constants can be obtained using equilibrium dialysis methods. Note that the maximum affinity constant occurs only when the antiserum is reacted against the hapten used in its production and that this constant rapidly declines, the further removed the chemically substituted molecules are that are used in reacting against the same antiserum. From H.N. Eisen, General Immunology, 3rd Edition, 1990, Lippincott, Philadelphia, p. 17.

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.

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