Critical Thinking

The term complement refers to a heterogenous group of at least 20 plasma proteins that normally circulate in an inactive state. Complement is activated by one of two pathways (classical or alternative) involving the plasma proteins. Each pathway involves a cascade in which complement proteins are activated in an orderly sequence leading to the cleavage of C3. Once C3b is bound to the target cell’s surface, it enzymatically
initiates the remaining steps of complement activation, which incorporates C5 through C9 (MAC) into the target cell membrane, ensuring lysis of the target cell. Other roles of complement include opsonization, inflammatory actions such as stimulating mast cells and basophils to release histamine (which increases vascular permeability), and attracting neutrophils and other inflammatory cells to the area.

The leaking of plasma proteins into the interstitial fluid causes an increase in the osmotic pressure, resulting in additional fluid leaking from the plasma and localized edema. This swelling dilutes the pathogen and sweeps any foreign material into the lymphatic vessels. The rush of fluid also delivers complement and clotting factors to the site of the injury.