Exam 3 Review:  Chapter 21:  Complement System

complement proteins - Any of the proteins in the complement system, which a system of 12 or more plasma proteins, designated C1, C2, etc., which react in an enzyme cascade* to a cell displaying immune complexes or to foreign cell surfaces; these proteins act in various combinations to coat the cell and promote phagocytosis (opsonization), make holes in the cell membrane (cytolysis), attract leukocytes to the area (chemotaxis), and otherwise enhance the inflammatory response.  [Note*:  two pathways to activation of the complement system are recognized, the classical and the alternate pathways.]

classical pathway - The primary or fundamental biochemical steps which activate the complement system, the complement proteins, which are triggered by the presence of an antigen-antibody complex; usually, the antibody is of the IgG or IgM class; the sequence of complement protein activation is C1, C2, C3 and C4 reacting to produce activated C3 which triggers the completion of the complement cascade*.  Follow this link for an excellent detailed explanation and animation.

alternate pathway - An additional set of biochemical steps which can amplify the activation of the complement system, the complement proteins, which are triggered by the presence of activated C3 ("C3b") on the surface of a cellular antigen (microbial polysaccharides); this additional sequence of complement protein activation is activated C3, Factor B, Factor P = Properdin, and Factor D reacting to produce additional activated C3 which triggers the completion of the complement cascade*.  Follow this link for an excellent detailed explanation and animation.

[Note*:  The completion of the complement cascade is a common set of steps in which C5, C6, C7, C8, and C9 are activated in sequence to produce the membrane attack complex.]

opsonization - To make bacteria or other cells more susceptible to phagocytosis (by neutrophils and monocytes/macrophages) by coating their cell membrane surfaces with various proteins, e.g., antibodies or elements of the complement system, e.g., activated C3 ("C3b").

chemotactic agent - Any chemical, usually a microbial protein or an immune regulatory substance, which induces the directed migration (but not the rate of movement) of leukocytes along its concentration gradient to the area(s) of higher concentration; histamine, various lymphokines, neutrophil products, and several of the activated complement proteins (C3a, C5a) serve this function.

cytolysis - The rupture and destruction of cells by the breakdown of their outer lipid cell membrane = plasmalemmae; various immune molecules have this ability including the membrane attack complex of the complement cascade (activated C5, C6, C7, C8, and C9).

membrane attack complex - The series of protein molecules achieving the completion of the complement cascade using a common set of steps in which C5, C6, C7, C8, and C9 are activated in sequence, by either the classical or alternate pathways, to produce the molecular structure capable of cytolysis -- one unit each of activated C5, C6, C7, and C8 ("C5b678") will bind from 10-16 molecules of C9 to assemble a hydrophobic pore (a very short tube) which crosses the cell membrane and lets ions, water, and other small molecules to pass in either direction, based upon concentration gradients; as a result, the attacked cell, which receives from hundreds to thousands of these pores from activation of the complement cascade, swells, ruptures, and dies.

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