Natural antibodies against a polysaccharide (Bo) from sugar cane mediate its complement-activating effect.

The interaction of a complement activating glucan from sugar cane (Bo) with immunoglobulins was studied. Bo precipitates a small fraction of IgG from human serum. In combination with this fraction, it activates complement by the classical pathway, not in its absence. Bo is adsorbed to and can be eluted from Sepharose containing covalently coupled IgG; it is not bound when the IgG-Sepharose is lacking the Bo-reactive IgG fraction. Bo-IgG complexes activate and fix C1. The binding of C1 to aggregated IgG is not inhibited by Bo. This is in contrast to the strong inhibition of C1 fixation by the Fc-binding protein A. It is concluded that normal human serum contains natural antibodies against Bo which form complement-activating immune complexes with Bo by binding it to their F(ab) region. The antibodies do not cross-react with dextran, a glucan from barley, nor with surface constituents of E. coli.