The overall objectives of the research project are to investigate the interaction of microbes, serum factors and phagocytic cells. We expect that it will be possible to identify microbial factors which are of key importance in opsonization and phagocytosis and to identify serum opsonin factors which react with these microbial structures and promote efficient phagocytosis. Finally, we anticipate that receptor mechanisms on phagocytic cells which react with serum opsonic factors will be identified. Patients with increased susceptibility to infection and newborn infants will be our primary patient population for study. The opsonic capacity of the patient's serum will be compared with control serum and the serum will be treated in a variety of ways. Heated serum will be used for evaluation of specific immunoglobulin opsonic activity. Serum chelated with MgEGTA will be used to examine alternative pathway opsonic activity. In newborn infants that receive plasma therapy, opsonic activity will be compared in their serum before and after therapy. The K1 E. coli strains from normal carriers will be compared with K1 E. coli strains recovered from blood and CSF for capacity to activate the alternative pathway and for virulence in the rat meningitis model. The role of circulating pneumococcal polysaccharide in host susceptibility will be evaluated by quantitating residual complement factors which remain after incubation of serum with polysaccharide. We have noted that very small quantities of activated C3 provide normal opsonic function therefore, the possible specific binding of C3 or C3 activator substance by pneumococcal polysaccharide and other bacterial factors will be examined.