Influence of surface hapten density of sheep red blood cells on the hemolytic or blocking activity of IgG antibodies.

The hapten density on the surface of sheep red cells, coated with benzylpenicilloic acid (BPO) was determined. This was achieved by comparison of the inhibitory capacity of the test cells with that of a standard cell preparation of known antigen density. IgG-induced hemolysis was much more affected by the recution of haptenic sites/cell than IgM-induced hemolysis. It was concluded that more than 18,000 protein islets on the sheep red cell surface bearing at least four haptenic sites were necessary for IgG-induced lysis. Target cells with 190,000 BPO groups/cell could be protected by anti-BPO-IgG against the lytic effect of simultaneously added IgM. Using target cells with 240,000 haptenic groups/cell or more a synergistic hemolytic action of IgG-and IgM-anti-BPO antibodies could be observed.