The complement system plays a crucial role in the innate defense against common pathogens. Activation of complement leads to robust and efficient proteolytic cascades, which terminate in opsonization and lysis of the pathogen as well as in the generation of the classical inflammatory response through the production of potent proinflammatory molecules. More recently, however, the role of complement in the immune response has been expanded due to observations that link complement activation to adaptive immune responses. It is now appreciated that complement is a functional bridge between innate and adaptive immune responses that allows an integrated host defense to pathogenic challenges.
On top of these important contributions to innate immunity, complement plays a vital role in shaping adaptive immune responses, functionally integrating it into the ability of the host to combat invasion from a wide range of pathogens. Since complement represents such an evolutionarily well-conserved mechanism of host defense, it is not surprising to find that it has been integrated into the relatively newer acquired immune responses. Complement has now been shown to play a role in both B- and T-cell responses at the organismal level. However, the exact mechanism(s) by which complement mediates T-cell immunity has yet to be determined. A careful, integrated study of complement effects on B- and T-cell biology will provide valuable insight into the in vivo biology of complement and may have implications for infectious disease as well as immunological disorders, such as in the cases of multiple sclerosis and organ transplantation.
In conclusion, complement is a multifaceted and robust effector, which bridges the innate and adaptive immune systems. It is vital to host defense, and the extent of its influence is becoming increasingly appreciated as additional information regarding the far-reaching effects of its activation is uncovered. Further study should produce significant dividends in our understanding of host defense as an integrated process and the roles complement plays in bridging innate and adaptive immunity.
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2. Carroll M C. (2004). The complement system in regulation of adaptive immunity. Nature immunology, 5(10), 981-986.