Interferons and complement are nonspecific antiviral and antibacterial substances. Interferons are small proteins produced by virus-infected cells. They are an important defense against infection by many different viruses. Complement is a group of 11 special proteins found in blood plasma. These proteins supplement or enhance (“complement”) certain immune, allergic, and inflammatory reactions.
Natural killer (NK) cells target and destroy abnormal or invading cells in the body. (From Rubin, E., M.D., and Farber, J. L., M.D. Pathology. 3rd Ed. Philadelphia: Lippincott, Williams & Wilkins, 1999.)