The complement system is made up of more than 25 plasma and cell membrane proteins, which are activated sequentially, and play important roles in immunologic effector mechanisms and host defense. Complement promotes bacterial, viral, and cellular lysis. Complement also acts to promote phagocytosis by the process of opsonization involving the deposition of specific complement fragments onto the surface of foreign microbes or cells. Receptors for these complement fragments are on phagocytic cells, leading to enhanced recognition and binding of the cell membrane to the phagocyte. Complement regulates inflammatory and immune responses, and has also been shown to play a role in acquired immunity because it helps to ensure the retention of antigen by follicular dendritic cells (FDC) in germinal centers, thereby providing a constant source of antigenic stimulus to activated, antigen-specific B cells.
Complement proteins can be grouped into four functional divisions according to their interaction with the third component (C3):
classic pathway activation,
alternative pathway activation,
the amplification mechanism, and
the effector mechanism.
An additional pathway of complement activation is the lectin pathway. Activation of this pathway is mediated by mannose binding lectin (MBL). The structure of MBL is similar to the C1q molecule of the classical complement pathway and is a pattern recognition receptor specific for mannose carbohydrates found only in bacteria and yeasts. MBL associates with two serine proteases (mannin binding lectin associated proteases 1 and 2 [ MASP-1 and -2]). This entire complex cleaves downstream components of complement.
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