Quantitation of immunoglobulin classes and subclasses of autoantibodies bound to red cells in patients with and without hemolysis.

The concentration of IgG, IgA, and IgM, as well as IgG subclasses, was measured by an enzyme-linked immunosorbent assay in autoantibodies eluted from red cells (RBCs); the number of molecules of each isotype per RBC was calculated. Three groups were analyzed: Group 1 included 23 patients with autoimmune hemolytic anemia (AIHA) associated with warm autoantibodies of IgG class; Group 2 included 11 patients without anemia but with a positive direct antiglobulin test (DAT); Group 3 included 10 healthy DAT-negative subjects. The mean number of IgG molecules per RBC in Group 1 (920) was about three times that in Group 2 (306) and about 17 times that in Group 3 (54). The range of RBC-bound IgG showed an overlap between the two groups of patients. The mean number of IgM and IgA molecules per RBC was low in the three groups. IgG1 predominated in all groups except in two patients with AIHA, in whom IgG3 made up at least 50 percent of total IgG. The mean number of IgG1, IgG2, and IgG4 molecules per RBC in Group 1 was about three times that in Group 2, whereas the mean number of IgG3 molecules per RBC was 10 times as high (p < 0.001). It follows that IgG3 was more common in patients of Group 1, but it was also detected in patients of Group 2.