A group of protective plasma proteins designated by the letter C and a number are all part of the system.

The group of protective plasma proteins designated by the letter C and a number are all part of the Complement system. The Complement system is a part of the immune system that enhances (complements) the ability of antibodies and phagocytic cells to clear microbes and damaged cells from an organism, promotes inflammation, and attacks the pathogen's cell membrane. It is named for its ability to "complement" the killing of pathogens by antibodies. Proteins in the complement system are named with a "C" and a number (like C1, C2, C3, etc.).