Mechanism of Red Cell Agglutination by IgG Antibodies

It has been shown by others that agglutination of red cells by IgC antibodies depends on the density of antigen sites at the red cell surface, suggesting the possibility that agglutination by IgG antibodies might possibly be due to the cells being rendered ‘sticky’ rather than to specific cross‐linking. However, the present experiments show that, when a mixture of purified IgG anti‐A and anti‐B is incubated with a mixture of A and B red cells, the clumps contain only A cells or only B cells, indicating that agglutination by these antibodies is, in fact, due to specific cross‐linking.