[23] Purification and quantitation of the components of the alternative complement pathway

Publisher Summary The alternative complement pathway, or properdin system, is known as a distinct mechanism in serum that bypasses the early-acting components of the classical sequence and activates the terminal complement sequence. A novel serum protein, termed “properdin,” was envisaged to cooperate with a hydrazine-sensitive Factor A (C3) and a heat-labile Factor B in the activation of complement. The activation of this pathway could be triggered by a variety of substances including microbial polysaccharides—such as zymosan, gram-negative bacterial lipopolysaccharides—certain mammalian cells—such as rabbit erythrocytes and lymphocytes—and some human lymphoblastoid cell lines. As the activity of this pathway appears not to require antibodies, it provides a natural or nonspecific defense against microbial infection and is probably the predominant pathway of complement activation in the early stages of infection, before synthesis of complement-fixing antibodies. The constituents of the alternative pathway of complement activation recognized to date include the third component of complement (C3), Factor B, Factor D, and magnesium ions. The pathway is stabilized by properdin and is regulated by the two control proteins, Factors H and I. The distinction between activators and nonactivators of the alternative pathway resides in the regulation of the functions of C3b and C3bBb by the control proteins, Factors H and I.