Overview:
Mohamed R Daha PhD, professor of Immunology, Leiden University Medical Center, The Netherlands.

The complement system is a major component of our innate immune defense and constitutes one of the key effector mechanisms of antibody-dependent and antibdy-independent immunity.The complement system is composed of a large number of molecules that are found in the circulation, in other body fluids and tissues in bilogicaly inactive state.
Biological activity of the complement system is generated following activation of the complement system via three intercomected pathways which ultimately lead to the activation of C3, the central component of complement, and generation of a large number of bilogical functions such as anaphylaxis, , chemotaxis and phagocytosis. The crucial function of C3 activation is evidenced by the susceptibilty of omplement deficient individuals to infections with various types of micro-organisms and the inabilty for proper handling of immune complexes and self debris.
In the transplantation setting complement plays a critical role in the protection of organ funtion after transplantation and in defense against microbial infections , especiallly because these patients have a suppressed aquired immune system because of treatmnent with immunosuprressive drugs. Under these conditions the patient is almost fully dpendent on the defensive properties of the innate immune sytem. Several studies have shown that especially the lectin pathway of complement is essential for proptection of the host against bacterial and fungal infections after liver and kidney transplantation.