Complement proteins are present in circulation and can be activated by a variety of mechanisms. Attached immunoglobulins can set off the "classic pathway" as shown above schematically. The complement cascade can generate active compounds such as C3b that functions as an opsonin, or C5a that attracts neutrophils. If the C5-9 complex (the "membrane attack complex") is generated, the cell to which the complex is attached can be lysed by perforation of its cell membrane.

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