The complement system

The complement system consists of a complex cascade of zymogens that leads to the formation of opsonins (predominantly C3b and C4b) that promote phagocytosis and the insertion of the membrane attack complex into the membranes, resulting in lysis. It constitutes one of the first lines of defense against pathogens as it does not require prior maturation or adaptation. The complement system also exerts an important influence on the adaptive immune response by acting synergistically with antibodies as well as promoting B- and T-cell stimulation. Although traditionally we have learned of three activation pathways, the reality is that there are multiple activation mechanisms provided by crosstalk with other systems such as the coagulation system. In order to prevent autologous attack, the complement system has many regulatory points that are intended to prevent autologous damage.