48] Alternative pathway of complement

Abstract

Alternative pathway provides an immediately available line of defense that does not require prior immunization. Because of a unique amplification process that occurs during activation, the alternative pathway generates large quantities of the anaphylatoxins C3a and C5a and deposits C3b on the surface of the activating particle C3b, and its breakdown products stimulate phagocytosis via receptors on neutrophils and mononuclear phagocytes. The alternative pathway is very sensitive to dilution and should be assayed with as little dilution as possible. Activity can rarely be detected at 20-fold or higher dilutions. The alternative pathway does not require Ca 2+ ions for activation, but it does require Mg2+.Perhaps, the most convenient assay for the activity of the human alternative pathway is lysis of rabbit erythrocytes. These cells directly activate the alternative pathway, and the release of hemoglobin is monitored to assess complement activity.