Serum complement depression during viral lower respiratory tract illness in cystic fibrosis.

Cystic fibrosis (CF) patients with viral lower respiratory tract illnesses (LRI) had depressed levels of the third and fourth components of complement, which returned to normal after recovery. There was no clinical evidence of immune complex disease. CF patients with LRI and no virus isolates, CF patients in stable status, and non-CF patients with LRI did not have complement depression. It is postulated that antigen-antibody complex activation of complement may occur in CF patients with viral LRI.