El sistema del complemento es uno de los componentes fundamentales de la conocida respuesta inmunitaria defensiva ante un agente hostil (por ejemplo, microorganismos). Consta de un conjunto de moléculas plasmáticas implicadas en distintas cascadas bioquímicas, cuyas funciones son potenciar la respuesta inflamatoria, facilitar la fagocitosis y dirigir la lisis de células incluyendo la apoptosis. Constituyen un 15% de la fracción de inmunoglobulina del suero.

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The complement system is a part of the immune system that enhances (complements) the ability of antibodies and phagocytic cells to clear microbes and damaged cells from an organism, promotes inflammation, and attacks membrane. It is part of the innate immune system, which is not adaptable and does not change over the course of an individual's lifetime. However, it can be recruited and brought into action by the adaptive immune system. The complement system consists of a number of small proteins found in the blood, in general synthesized by the liver, and normally circulating as inactive precursors (pro-proteins). When stimulated by one of several triggers, proteases in the system cleave specific proteins to release cytokines and initiate an amplifying cascade of further cleavages. The end result of this complement activation or complement fixation cascade is stimulation of phagocytes to clear foreign and damaged material, proxy inflammation to attract additional phagocytes, and activation of the cell-killing membrane attack complex. Over 30 proteins and protein fragments make up the complement system, including serum proteins, serosal proteins, and cell membrane receptors. They account for about 10% of the globulin fraction of blood serum and can serve as opsonins. Three biochemical pathways activate the complement system: the classical complement pathway, the alternative complement pathway, and the lectin pathway.

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