a normal serum α-globulin (MW about 25,000) required in the properdin system to cleave factor B into Bb and Ba. Bb combines with C3b to form the C3 convertase of the alternative complement pathway.
pro·per·din fac·tor D
(prō'pĕr-din fak'tŏr) A normal serum α-globulin required in the properdin system.