Autoimmune haemolytic anaemia in childhood associated with non-complement binding IgM autoantibodies.

Red cell bound IgM autoantibodies free of associated IgG autoantibodies and unable to activate complement have not yet been considered as the cause of autoimmune haemolytic anaemia (AIHA). We report on warm type AIHA resulting from the action of IgM autoantibodies on circulating RBC. Twelve children (eight of whom were infants) with relatively severe haemolytic anaemia were studied. Whereas the clinical findings and courses of all children appeared to be compatible with warm type AIHA, the serological findings during the haemolytic phase were atypical in that the direct antiglobulin test (DAT) was negative in 11, and positive in one case due to C3d only. The use of radiolabelled antihuman globulin antibodies showed, however, that the RBC of all the patients were sensitized with warm IgM antibodies. Elevated values of IgA and/or IgG immunoglobulins on the patients' RBC were found to be present in only two cases. Complement activation by IgM autoantibodies could not be detected in all other cases neither in vivo nor in vitro. Thus, we conclude that non-complement binding IgM autoantibodies were responsible for the AIHA in the majority of these children possibly representing an as yet unrecognized variant of warm type AIHA.