Antibiotic Modulation of Host Defense

Abstract

Infection is a major cause of morbidity and mortality in man. Many gram-negative bacteria are sensitive to the bacteriolytic activity of serum alone. When bacteria are not lysed by serum, as is common for gram-positive microorganisms, they usually become opsonized and subsequently phagocytized by phagocytic cells and rapidly killed. Optimal phagocytosis requires the participation of opsonins, including serum complement and immunoglobulin (Ig). Although opsonization with complement is primarily mediated through activation of complement via IgG at the bacterial cell surface, these factors may also become fixed to microorganisms without the participation of antibody. Receptors on polymorphonuclear leukocytes (PMNs) with specificity for C3b and C3bi (the activated third complement components) and for the Fc fragment of the antibody molecule have been described, and evidence suggests that these receptors play a major role in phagocytosis of staphylococci opsonized in normal serum.