Low molecular weight IgM in sera of children following bone marrow transplantation for severe aplastic anaemia and acute leukaemia.

Serum samples of 16 children, treated with allogeneic bone marrow grafts for either aplastic anaemia or leukaemia, were investigated longitudinally for the presence of pentameric (19S) and low molecular weight (LMW) (7S) IgM. From 2 months following grafting, serum IgM increased in all patients, gradually in seven cases and with a transient overshoot in the other nine. In eight patients, LMW-IgM was detected transiently in the serum within the period when total IgM was increasing, either with (five patients) or without (three patients) overshoot. These data indicate that, at a certain stage of the immunological reconstitution after transplantation as in normal ontogeny, rather immature cells of the B cell lineage are able to produce and secrete LMW-IgM.