ABSTRACT
The complement system is comprised of a series of glycoproteins found circulating in the blood in inactive precursor forms. Activation of complement involves a sequential cascade of enzymatic (primarily proteolytic) reactions that produces the active forms of the complement proteins. Activation of the complement cascade sets in motion a series of events that facilitate the extravasation of mononuclear phagocytes into areas of infection, destroy invading microorganisms by opsonization or by direct membrane attack complex-mediated membrane damage, and clear the blood of antigens such as bacteria and bacterial debris by routing C3b-bound antigens to the liver and spleen for ingestion by hepatic and splenic macrophages. Biochemical and electron microscopy studies have shown that during growth in enriched media, enteric bacteria release lipopolysaccharide-, phospholipid-, and membrane protein-containing vesicles into the media. However, lipopolysaccharide release may be markedly increased when bacteria are incubated with human serum.
