The complement system is a component of the innate immune system, which consists of physical, cellular, and chemical elements. The immune system has evolved to protect the human body against pathogens or other dangerous elements, and its responses can be divided into innate and adaptive immune responses. Traditionally, the main function of the complement system was believed to be limited to the recognition and elimination of pathogens through direct killing and/or stimulation of phagocytosis (innate responses).
In recent years, the immunoregulatory functions of the complement system were demonstrated and it was determined that the complement proteins play an important role in modulating adaptive immunity and in bridging innate and adaptive responses. The contribution of complement to the development of humoral immunity has been confirmed through a series of elegant studies, and a body of data has accumulated demonstrating that the activation of the complement system is also critical to the development of T cell immunity.
When the delicate mechanisms that regulate this sophisticated enzymatic system are unbalanced, the complement system may cause damage, mediating tissue inflammation. Dysregulation of the complement system has been involved in the pathogenesis and clinical manifestations of several autoimmune diseases, such as systemic lupus erythematosus, vasculitides, Sjogren's syndrome, antiphospholipid syndrome, systemic sclerosis, dermatomyositis, and rheumatoid arthritis. Complement deficiencies have been associated with an increased risk to develop autoimmune disorders. Because of its functions, the complement system is an attractive therapeutic target for a wide range of diseases.
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