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23.6: Complement System

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    An array of approximately 20 types of soluble proteins, called a complement system, functions to destroy extracellular pathogens. Cells of the liver and macrophages synthesize complement proteins continuously; these proteins are abundant in the blood serum and are capable of responding immediately to infecting microorganisms. The complement system is so named because it is complementary to the antibody response of the adaptive immune system. Complement proteins bind to the surfaces of microorganisms and are particularly attracted to pathogens that are already bound by antibodies. Binding of complement proteins occurs in a specific and highly regulated sequence, with each successive protein being activated by cleavage and/or structural changes induced upon binding of the preceding protein(s). After the first few complement proteins bind, a cascade of sequential binding events follows in which the pathogen rapidly becomes coated in complement proteins.

    Complement proteins perform several functions. The proteins serve as a marker to indicate the presence of a pathogen to phagocytic cells, such as macrophages and B cells, and enhance engulfment; this process is called opsonization. Opsonization refers to an immune process where particles such as bacteria are targeted for destruction by an immune cell known as a phagocyte. Certain complement proteins can combine to form attack complexes that open pores in microbial cell membranes. These structures destroy pathogens by causing their contents to leak, as illustrated in Figure 1.

    Illustration shows an invading pathogen with an antigen on its surface. In the classic pathway for complement activation, host antibodies bind the antigen, and C1 binds the antibody. The C1-antibody complex causes C2 and C4 each to split in two. Fragments from C2 and C4 each joins together to form an enzyme called C3 convertase. C3convertase splits C3 in two. One of the fragments from C3 joins C3 convertase to form C5 convertase. C5 convertase splits C5 in two. A fragment from C5 joins C6, C7, C8, and C9 to form a complex that makes a hole in the plasma membrane for the invading cell. The cell swells and bursts. In the alternative pathway, C3 convertase spontaneously splits C3 in two and the rest of the pathway proceeds the same as the classic pathway. Host cells are protected from complement by the presence of endogenous proteins.
    Figure 1. Click for a larger image. The classic pathway for the complement cascade involves the attachment of several initial complement proteins to an antibody-bound pathogen followed by rapid activation and binding of many more complement proteins and the creation of destructive pores in the microbial cell envelope and cell wall. The alternate pathway does not involve antibody activation. Rather, C3 convertase spontaneously breaks down C3. Endogenous regulatory proteins prevent the complement complex from binding to host cells. Pathogens lacking these regulatory proteins are lysed. (credit: modification of work by NIH)

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