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

The method of preparation of partially purified [I]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 RR. 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 () of two different examples of IgM anti-D were 1.7×10 and 2.5×10 litres/mole, and of the IgG antibodies obtained from the same plasmas were 9- and 32-fold lower respectively. The 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.