Human monocyte binding of homologous monomer and complexed IgG

Human peripheral blood monocyte binding of homologous IgG has been studied using monomer and covalently cross-linked dimer and trimer. A competitive radioassay using enriched monocytes was employed to determine the association constant (Ka) and the number of receptor sites per cell. Monomer and dimer bound with very similar Ka values (7.15 X 10(7) and 7.14 X 10(7)LM-1) whereas a slightly higher figure was obtained with trimer (8.9 X 10(7)LM-1). The number of receptor sites for dimer and trimer (90.5 and 107.5 X 10(3)/cell, respectively) were much greater than for monomer (14.5 X 10(3)/cell). Inhibition of dimer and trimer binding by monomer gave heterogenous Scatchard plots, each with two components. These findings suggest that human peripheral blood monocytes possess two types of Fc receptor, one of high affinity binding monomer and complexes and one of low affinity which predominantly binds complexes.