Briefly describe the complement system, including the role of C4BP (C4b-binding protein).
The complement system is a complex system of serum proteins which interact in a cascade as part of the immune system. Complement helps antibodies kill invaders. Complement proteins mark any cells which do not have certain protective proteins on their outsides. The markers attract phagocytes (white blood cells that digest other cells) to destroy the invader, or can break the cell membrane of the invader. The classical pathway is activated by antibody-antigen complexes. C1-esterase is activated primarily by immune complexes containing IgM or IgG antibodies. The alternative pathway is initiated when a previously activated complement component (C3b) binds to the surface of a pathogen, where it is protected. Hence, the alternative pathway can be activated directly by foreign cell surfaces. Both pathways lead to the formation of a self-assembled membrane-attacking complex, which binds to foreign cell surfaces and induces lysis of these cells. C3 convertases are the key players in complement activation.