Role of humoral factors in host resistance to the Bacteroides fragilis group.

The alternative complement pathway and antibody play an important role in host resistance to members of the Bacteroides fragilis group. Clinical isolates of the B. fragilis group are typically resistant to the bactericidal action of serum. Naturally occurring antibodies in serum directed against these bacteria belong primarily to the IgM class. These antibodies are not required for activation of the alternative pathway by the bacteria but rather potentiate alternative pathway activation. Activation of the alternative pathway by the bacteria leads to the generation of chemotactic activity for neutrophils and to C3 deposition on the bacterial surfaces. Both iC3b, the predominant molecular form of the bacteria-bound C3, and IgM antibody are necessary for phagocytosis and killing of the bacteria by neutrophils. IgM acts synergistically with iC3b to facilitate adherence of the bacteria to the neutrophils. Certain Bacteroides strains require additional as yet uncharacterized auxiliary opsonins for maximal adherence to neutrophils.