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properdin, protein found in the blood serum of humans and some of the higher animals that appears to participate in certain specific immune responses. It is associated with the engulfing of foreign particles and invading cells by phagocytes and with tissue inflammation (see blood; immunity). Properdin has been isolated in highly purified form.
Properdin or factor P is a globulin protein found in the blood serum of higher 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.

The alternative pathway is not dependent on antibodies. This branch of the complement system is activated by IgA immune complexes and bacterial endotoxins, polysaccharides, and cell walls, and results in producing anaphylatoxins, opsonins, chemotactic factors, and the membrane attack complex, all of which help fight pathogens.


Properdin was discovered in 1954 by Dr. Louis Pillemer of the Institute of Pathology (now the Department of Pathology at Case Western Reserve University).

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