Properdin, or factor P, is a single-chain plasma glycoprotein (apparent molecular mass 52-55 kDa), consisting of six thrombospondin repeat sequences between short N- and C-terminal domains. In the blood it exists as a mixture of head-to-tail dimers, trimers and tetramers, with a preponderance of trimers.The protein is produced by a variety of leukocytes, including monocytes, T lymphocytes and neutrophils , but also by endothelial cells in which properdin synthesis and release are induced by shear stress.Properdin participates in the alternative pathway of complement activation together with C3 and factors B, D, I and H. It stabilizes the labile C3bBb which is deposited on immune complexes or foreign surfaces. This permits amplification of C3bBb formation in competition with catabolism of C3b by factor I, which uses factor H as a cofactor. The local amplification process leads to the creation of the alternative pathway C5 convertase, C3bBb3b, and initiates the terminal pathway of complement activation.Properdin prolongs the half-life of surface-bound C3bBb from 1½ minute to about 18 minutes. This is due to several effects: inhibition of C3b cleavage by factor I, increased affinity for factor B and inhibition of the dissociation of C3bBb into C3b and Bb. Properdin is consumed by binding to C3bBb, this binding showing an order of preference of tetramers over trimers over dimers, which is also the order of functional activity of the oligomeric forms.Deficiency or malfunction of the molecule may lead to severe impairment of alternative pathway activation, depending on the precise nature of the defect. One parameter of functional defect in the presence of measurable levels of protein is an impaired generation of the more active tetramer and trimer forms.The properdin gene is located on the short arm of the X chromosome, and congenital properdin deficiencies are therefore inherited as typical X-linked recessive traits.Three types of familial deficiency have been described: type 1 (or I) is characterized by serum with very low or absent properdin activity in hemolytic assays and <0.1 μg/ml="" immunoreactive="" protein;="" type="" 2="" (or="" ii)="" is="" characterized="" by="" low="" but="" detectable="" levels="" of="" immunoreactive="" protein="" (»2="" μg/ml)="" and="" impairment="" of="" some,="" but="" not="" all="" functional="" test,="" and="" type="" 3="" (or="" iii)-="" has="" normal="" levels="" of="" immunoreactive="" but="" dysfunctional="" protein.="" 10="" normal="" serum="" or="" plasma="" levels="" of="" properdin="" are="" variously="" quoted="" as="" about="" 5="" or="" 25="" µg/ml.="" differences="" in="" properdin="" levels="" between="" healthy="" and="" diabetic="" persons="">