The interaction of antigen with specific, cell-associated receptors was measured in thermodynamic terms. The binding of 125I-labeled 2,4-dinitrophenyl guinea pig albumin (DNP16GPA-125I) to lymphocytes from guinea pigs immunized to DNP16GPA is a temperature-dependent, reversible process. Measurement of association and dissociation rates of antigen-receptor complexes permits calculation of antigen-cell binding constants. These may also be calculated by equilibrium-binding techniques. Although differences in the constants calculated in these two ways exist, a clear increase in avidity of cell receptor for antigen occurs in the course of the immune response. This change in receptor avidity provides evidence that the time-dependent change in affinity of serum antibody (maturation) indeed has a cellular basis.
The magnitude of the equilibrium constant is, in part, due to binding of more than one DNP group per molecule of antigen. Thus, multivalent ligands bind more effectively to cell receptors than univalent or paucivalent ligands when measured by the number of antigen molecules bound, the dissociation rate of antigen-receptor complexes, and in the relative capacity to inhibit a standard multivalent ligand (DNP16GPA-125I) from binding.