April 11, 2012
The Complement System
The complement system is a nonspecific defense mechanism activated in response to invading organisms. This system gets its name from the fact that it complements the action of antibodies. It is the primary mechanism activated by antibodies to kill foreign cells by forming membrane attack complexes. In addition to directly lysing the invading organisms it also reinforces the general inflammatory tactics. It can be activated in two ways: 1. A nonspecific innate immune response. This is when there is exposure to particular carbohydrate chains present on the surface of micro-organisms, but not found on humans cells. 2. An adaptive immune response. This is when there is exposure to antibodies produced against a specific foreign invader.
The complement system consists of plasma proteins that are produced by the liver and circulate in the blood in inactive form. Once the first component, C1 is activated it activates the next component, C2 and so on in a sequential chain of activation reactions. The final five components, C5 – C9, assemble into a large donut shape protein complex called the membrane attack complex (MAC). The MAC attacks the surface of the membrane of the micro-organism by imbedding itself through the microbial surface membrane and literally creating a hole in the surface. This creates a leaky surface allowing an osmotic flux of water into the cell causing it to swell and burst.
The complement system augments inflammation in several ways once activation occurs. It serves as chemotaxins, which guides phagocytes to the site of microbial invasion. The system acts as opsonins, which binds with microbes and thereby enhances phagocytosis. It will promote vasodilation and increased vascular permeability to increase blood flow to the invaded area. In addition, it stimulates the release of histamine from mast cells to enhance the local vascular changes characteristic...