Membrane attack complex is -

Components of complement cascade mediate various biological responses  C5a, C3a, to a lesser extent C4a → are called anaphylotoxin because they have effects similar to those of mast cell mediators that are involved in the reaction called anaphylaxis. These effects are due to stimulation of histamine release by this anaphylatoxin. The effects are increased vascular permeability and vasodilatation.  C5a is a powerful chemotactic agent  C3b, C3bi act as an opsonin and enhance phagocytosis.  C called membrane attack complex (MAC). It forms a channel in the lipid membrane and causes cell lysis (including bacteria).