Glycoprotein complement factor C9 is synthesized as a single chain protein of 558AA and is present in human serum at ca 60ug/ml. C9 is predominantly formed in the liver, but is also synthesized by monocytes, macrophages, fibroblasts and glial cells. Pore forming proteins with a membrane-attack complex perforin domain (MACPF) are most effective to kill bacteria or virus infected cells. The MACPF domain polymerizes, refolds and inserts itself into the bilayer membrane. C5b binds to the membrane and binds C6&7. The addition of C8 to the complex triggers and accelerates C9 polymerization in order to form the asymmetric pore on the target surface. A complete pore can exist out of 12-18 C9 monomers. Together with CD59, MAC formation is regulated by eg DAF, MCP and CR1. C9 deficiency is quite common, especially in the Japanese population. These patients are more susceptible for bacterial infection.