The limited fragmentation of porcine haemolytic immunoglobulin M. Evidence that haemolytic activity depends on the number of Fab arms that can attach to the erythrocyte surface.

Methods previously developed in this laboratory for the proteolytic removal of limited numbers of Fab arms from the IgM molecule have been used to study haemolytic activity. The gradual removal of arms produced a rapid decrease in complement-mediated lysis of erythrocytes but had a much slower effect on their agglutination. Therefore a stage was reached at which molecules with four Fab arms gave good agglutination but did not cause cell lysis. The same result was obtained whether Fab arms were removed randomly by pepsin or in pairs by trypsin. Hence the non-lytic nature of these molecules was independent of the arrangement of their remaining Fab arms. A sharp decrease in haemolytic activity was also obtained under conditions of high antibody concentration when the IgM molecular could attach only a few of its Fab arms to the erythrocyte surface. Possible explanations for the dependence of haemolytic activity on the number of Fab arms discussed.