The complement system comprises a far-reaching and vital component of innate immunity and represents one of the major effector mechanisms of the innate immune system. Discovered in 1896 by Bordet as a heat-labile component of serum, it was so named for its ability to 'complement' the antibacterial properties of antibody in the heat-stabile fraction of serum. It is now appreciated that complement is a complex network of plasma and membrane-associated serum proteins which can elicit highly efficient and tightly regulated inflammatory and cytolytic immune responses to infectious organisms (bacteria, viruses, parasites), tissue damaged by physical, chemical, or neoplastic insults, and other surfaces identified as 'nonself'.
For many years after its discovery, the role of complement system in immunity was thought to be confined to innate immune responses with no impact on adaptive immune responses, in much the same way as innate immunity, in general, was relegated to those functions of immunity that involved prevention and confinement of infection while adaptive immunity provided effectors required to clear the infection. The ability to separate the functions of the two arms of immunity was called into question as early as the 1970s, and since then the body of knowledge illustrating the interplay between the adaptive and innate wings of immunity has grown dramatically. Similarly, the ability of complement to not only affect robust innate immune responses but also to interface with and influence T- and B-cell biology and adaptive responses has become increasingly appreciated. This review attempts to summarize the roles that complement biology plays in the immune response, both in the innate detection and elimination of pathogenic infections and in the modulation of adaptive immune responses.
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