Role of complement in innate immunity and infections

The complement system is a major player in our innate defense and constitutes one of the key effector mechanisms of antibody-dependent and -independent immunity. The complement system is composed of a large number of proteins found in the circulation, in other body fluids, and in tissues in a biologically inactive state. Biological activity of the complement system is generated following activation of the complement system via three initiating pathways, which ultimately lead to the activation of C3, the central component of complement, and the generation of a large number of biological functions such as anaphylaxis, chemotaxis, and phagocytosis. The crucial role of C3 activation is evidenced by the susceptibility of individuals with C3 deficiency to infections with various types of microorganisms and the inability to properly handle immune complexes, self-debris, etc. This review will focus on the protective role of the lectin pathway of complement and the alternative pathway in bacterial defense.