The membrane attack complex (MAC) is a structure formed by the complement system, a part of the immune system. It plays a crucial role in defending the body against infections by creating pores in the membranes of target cells, such as bacteria. This process leads to cell lysis, or destruction, which helps eliminate pathogens. The MAC is composed of several proteins, primarily C5b, C6, C7, C8, and multiple C9 molecules. Once activated, these proteins assemble on the surface of the target cell, forming a channel that disrupts the cell's integrity. This action is vital for the immune response, aiding in the clearance of harmful microorganisms.