Properdin or factor P is a globulin protein found in the blood serum of more complex animals. In the complement system, an innate-immunity series of proenzymes dissolved in the circulation, it is also called "Factor P". It is known that it participates in some specific immune responses. It plays a part in tissue inflammation as well as the engulfing of pathogens by phagocytes. In addition it is known to help to neutralize some viruses.
As a component of the alternative pathway for complement activation (otherwise known as the "properdin pathway"), it complexes with another protein, C3b, to stabilize the alternative C3 convertase (C3bBb) that then cleaves more C3.Properdin was discovered in 1954 by Dr. Louis Pillemer of the Institute of Pathology .