How is the complement system activated?

The complement system comprises plasma proteins that can be triggered directly by pathogens and injuries or indirectly by pathogen-linked antibodies. This initiation leads to a series of reactions on the surface of pathogens, generating active components with various functions. Three pathways of complement activation exist: the classical pathway (activated by pathogens or antibodies bound to them), the MB-lectin pathway, and the alternative pathway (which amplifies the other pathways). All three can start without antibodies as part of the innate immune response. The iInitial stages involve sequential cleavage reactions where the resulting fragments bind to the pathogen's surface, activating the next component. These pathways merge when a C3 convertase enzyme is formed, splitting C3 to form C3b. The attachment of numerous C3b molecules to the pathogen is the key event in complement activation.