The term complement refers to the ability of these proteins to assist, or complement the antimicrobial activity of antibodies. The complement system is a collection of circulating and cell membrane proteins that play important roles in host defense against most bacterial infections. The complement system may be activated by microbes in the absence of antibody, as part of the innate immune response to infection, and by antibody attached to microbes, as part of adaptive immunity.
The complement system consists of about 20 interacting soluble proteins that work together to assist the acction of antibodies in destroying bacteria. They are made mainly by the liver and circulate in the blood and extracellular fluid. Most are inactive until they are triggered by an immune response or by an invading microorganism itself. Individuals with a deficiency in one of the central complement components (called C3) are subject to repeated bacterial infections, just as are individuals deficient in antibody themself.
The three biochemical pathways acivate the complement system are the classical complement pathway, the alternative complement pathway and the lectin pathway. In both classical and alternative pathways, the reaction of complement activation usually take place on the surface of an invading mircobe, such as a bacterium. C1 – C9 and factors B and D are reacting component s of the complement system.
The classical pathway which is initiated by antigen-antibody complexes, is usually activated by clusters of IgG or IgM antibodies bound to antigens on the surface of a microorganism. The first step in this pathway, is activated by polysaccharides in the cell envelopes of microorganism. Even in the absence of antibody, although activation of the classical pathway also activates the alternative pathway through a positive feedback loop.
The alternative pathway, which is triggered by other means, including invading pathogen or tumor cell, provides a first line of...