Peripheral blood mononuclear cell. In this study, there was a significant association between being in group MB0 and splenomegaly, lymphoid proliferation and granulomatous disease, although autoimmune disease was found with equal prevalence in all three groups. Four patients from the MB0 group were analyzed for defects in somatic hypermutation of the VH genes. In these patients, isotype-switched memory B cells were found to have normal somatic mutations when compared with controls. Splenomegaly and granulomatous disease were associated with the group with decreased switched memory B cells.