Observations on the reactions between D-positive red cells and ( 125 I)IgM anti-D molecules and subunits

Abstract

The method of preparation of partially purified [¹²⁵I]IgM anti-D and of subunits of 180,000, 90,000 and 40,000 molecular weight with anti-D activity is described. It was found that approximately 11,500 whole IgM anti-D molecules would bind to each red cell of phenotype R₁R₂. On the other hand, approximately 31,000 molecules of both whole IgG anti-D and an IgM anti-D subunit of molecular weight 40,000 would bind to the same cell. The following possibilities are proposed to explain these findings: (1) IgM anti-D molecules bound to the red cell sterically hinder binding of other IgM antibody molecules to closely adjacent D-antigen sites and (2) some D-antigen sites are too far below the surface of the red cell to permit binding by the large IgM molecule.

The average values of the equilibrium constants (K₀) of two different examples of IgM anti-D were 1.7×10⁹ and 2.5×10⁹ litres/mole, and of the IgG antibodies obtained from the same plasmas were 9- and 32-fold lower respectively. The K₀ of the IgM subunits was on average 23-fold lower than that of the whole IgM and it is concluded that IgM anti-D binds bivalently to each red cell but that the binding energy of one of the sites is relatively small.