The alternative pathway of the complement system is a humoral component of the immune system's natural defence against infections which can operate without antibody participation. The cascade begins with the spontaneous activation and attachment of the complement protein C3 to a pathogen, creating a catalytic activator that decays both spontaneously and quickly, but can lead to the creation of a C3 convertase. The alternative pathway convertase consists of the activated B and D factors, forming an unstable compound that can become stable after binding properdine, a serum protein. After the creation of C3 convertase, the complement system follows the same path regardless of the means of activation (alternative, classical, or MBL). The complement system is a complex biochemical cascade of the immune system, leading to cytolysis, chemotaxis, opsonization and inflammation, it can mark pathogens for phagocytosis. ... The immune system is the organ system that protects an organism from outside biological influences. ...
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