Complement activation pathways have evolved to label pathogens for elimination. The classical pathway links to the adaptive immune system through antibodies. The alternative and lectin pathways provide antibody-independent ‘innate’ immunity, and the alternative pathway is linked to and amplifies the classical pathway.

The complement system is carefully controlled to protect the body from excessive or inappropriate inflammatory responses. C1 inhibitor controls the classical and lectin pathways. C3 and C5 convertase activity is controlled by decay and enzymatic degradation. Membrane attack is inhibited on host cells by CD59.

The membrane attack pathway results in the formation of a lytic transmembrane pore. Regulation of the membrane attack pathway by CD59 reduces the risk of ‘bystander’ damage to adjacent host cells. Complement has a variety of functions. Its principal functions include opsonization, chemotaxis and cell activation, lysis of target cells, stimulation of inflammation, clearance of immune complexes and priming of the adaptive immune response.