Properdin: approaching four decades of research.

Properdin plays a key role in the alternative pathway by stabilizing the C3 convertase. This review chronicles the evolution of our knowledge of the structure and function of properdin over the 40 years since its discovery. The story begins with the first description of properdin and the controversy that ensued over its existence. Then, this review describes: the rediscovery and acceptance of properdin and the alternative pathway as important components of natural immunity; properdin structure and molecular sequence; speculations on the significance of sequence similarities to thrombospondin and other proteins, and properdin biosynthesis. Finally, this review characterizes properdin deficiency. Properdin does indeed appear to play an important role in homeostasis.