IgG2 class red cell antibodies and autoimmune haemolysis

To examine the role of IgG2 red cell autoantibodies in autoimmune haemolysis.

Study of immunohaematology case records.

Six patients had only IgG2 autoantibodies detected by direct antiglobulin testing and in red cell eluates; two individuals, whose red cells were also coated with complement, suffered from autoimmune haemolytic anaemia.

IgG2 antibodies are found alone in <1% of patients with warm autoantibodies and even more rarely cause red cell destruction. Several factors are important for inducing haemolysis. They include allele differences in the FCRIIA genes encoding for the FcyRII receptors--an allele with high affinity for IgG2 is needed for haemolysis. Topography of red cell antigens may also be significant; IgG2 is a relatively inflexible molecule and access of effector cell Fc receptors to the recognition sites on the IgG2 might be impossible unless the antigens are on, or proud to, the red cell surface. On rare occasions, IgG2 activates complement (as in our patients with active haemolysis); the synergistic effect between red cell bound immunoglobulins and C3 in causing haemolysis is well recognised.