While the antigen-antibody interaction can be described by the conн centration and affinity of the two reactants, the recognition of exogenous protein antigens by class II-restricted T cells follows more complex rules. T lymphocytes recognize complexes of antigen bound to major histoн compatibility complex (MHC) molecules on the surface of antigen preн senting cells (APC). Since most soluble protein antigens do not bind in their native form to class-II molecules, the APC have to capture these antigens, then internalize and process them, usually by proteolysis, into a form able to bind to class-II molecules. These complexes are reexposed on the cell surface where they must reach a critical threshold for T-cell triggering (1).

Thus, the concentration of the antigen-MHC class-II complexes that accumulate on an APC exposed to antigen is dependent on several factors:(a) the concentration of free antigen,(b) the efficiency of antigen capture by APC,(c) the efficiency of processing,(d) the concentration of class-II molecules with available binding sites and,(e) the stability of the complex.