Constituent of the membrane attack complex (MAC) that plays a key role in the innate and adaptive immune response by forming pores in the plasma membrane of target cells. C9 is the pore-forming subunit of the MAC. Defects in C9 are a cause of complement component 9 deficiency (C9D). A rare defect of the complement classical pathway associated with susceptibility to severe recurrent infections, predominantly by Neisseria gonorrhoeae or Neisseria meningitidis. Belongs to the complement C6/C7/C8/C9 family. Note: This description may include information from UniProtKB.