c9

activated ninth component of complement

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a component of the membrane attack complex; multiple molecules are bound by a single C5b,6,7,8 complex to form the final membrane attack complex. Although as few as 3 to 4 molecules of C9 are sufficient for full lytic activity on many substrates, 12 to 15 are necessary to form the usual doughnut-shaped pore in the membrane. C9_n is often used to denote the multiple molecules of C9 in the membrane attack complex.