The alternate pathway of complement activation. The role of C3 and its inactivator (KAF).

The immunochemical depletion from serum of C3b inactivator (KAF) has been performed using purified F(ab′) antibody. KAF depletion leads to spontaneous activation of the `alternate pathway of complement fixation' as evidenced by conversion of glycine-rich β-glycoprotein, depletion of cobra venom factor—C3 proactivator and conversion of C3. The status of the complement system following KAF depletion accurately mimics that found in the unique KAF-deficient patient. The activation of the alternative pathway whether by KAF depletion or by conventional alternative pathway activators is prevented by the immunochemical depletion of C3. It therefore appears that the alternative pathway is essentially a C3b-feedback pathway which is normally controlled by the activity of KAF.