Opsonins: Their function, identity, and clinical significance

The phagocytic process is composed of four interrelated phases: chemotaxis,opsonization, ingestion, and digestion. The function of opsonins is to react with bacteria and make them more susceptible to ingestion by phagocytes. Opsonization of bacteria may occur by three different mechanisms. First, specific antibody alone may act as an opsonin. Specific antibody may also act as an opsonin in concert with complement, by activating C3 via the classic pathway of C1, C4, and C2. Finally, there is a nonspecific mechanism of opsonization present in nonimmune animals called the heat-labile opsonin system. In the heat-labile opsonin system, 7S immunoglobulin from nonimmune animals activates C3 via an alternate pathway distinct from C1, C4, and C2. These different mechanisms may operate concurrently in vivo, but one mechanism may be favored over another depending on the type of organism, the stage of the infection, and the presence or absence of specific antibody. There are a number of different clinical situations in which an increased susceptibility to infection is associated with decreased serum opsonizing activity. These situations serve to illustrate the important role opsonins play in the host's resistance to infection.