Alternate Pathway - surface leading to cleavage of C3....

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Alternate Pathway The alternate pathway may be initiated by immunologic (e.g. IgA or IgE) or non-immunologic (e.g. LPS) means. The cascade begins with C3. A small amount of C3b is always found in circulation as a result of spontaneous cleavage of C3 but the concentrations are generally kept very low (see below). However, when C3b binds covalently to sugars on a cell surface, it can become protected. Then Factor B binds to C3b. In the presence of Factor D, bound Factor B is cleaved to Ba and Bb; Bb contains the active site for a C3 convertase. Next. properdin binds to C3bBb to stabilize the C3bBb convertase on cell
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Unformatted text preview: surface leading to cleavage of C3. Finally, a C3bBb3b complex forms and this is a C5 convertase, cleaving C5 to C5a/b. Once formed, C5b initiates formation of the membrane attack complex as described above. Generally, only Gram-negative cells can be directly lysed by antibody plus complement; Gram-positive cells are mostly resistant. However, phagocytosis is greatly enhanced by C3b binding (phagocytes have C3b receptors on their surface) and antibody is not always required. In addition, complement can neutralize virus particles either by direct lysis or by preventing viral penetration of host cells....
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This note was uploaded on 11/05/2011 for the course BIOLOGY MCB2010 taught by Professor Jessicadigirolamo during the Fall '10 term at Broward College.

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