Factor D cleaves factor B when the latter is complexed with factor C3b, activating the C3bbb complex, which then becomes the C3 convertase of the alternate pathway. Its function is homologous to that of C1s in the classical pathway.
Defects in CFD are the cause of complement factor D deficiency (CFD deficiency) [MIM:134350]. CFD deficiency predisposes to invasive meningococcal disease.
Belongs to the peptidase S1 family. Contains 1 peptidase S1 domain.