An assembly of complement proteins that creates a pore in the plasma membrane of a pathogen, thereby contributing to its destruction. It is one of the mechanisms of innate immunity that follows activation of the complement cascade. Certain complement components (C5b, C6, and C7) bind to the lipid bilayer of the target cell, insert into it, and then induce polymerization of the C9 component to form a cylindrical channel about 10 nm in diameter. This disrupts the membrane’s integrity and electrochemical properties and kills the cell. The MAC is significant for defence against only certain bacterial infections, such as Neisseria spp. responsible for gonorrhoea, but is important in certain immune system diseases.