C4-binding protein in sera of patients with systemic lupus erythematosus and mixed essential cryoglobulinemia.

C4-binding protein (C4BP) concentration was measured in sera of patients with systemic lupus erythematosus (SLE) (59) and mixed essential cryoglobulinemia (MEC) (6). The mean concentration of C4BP was not significantly different from the normal controls in both groups of patients; 1 patient with MEC and 11 patients with SLE had concentrations below the normal range. In addition there was no significant correlation between C4BP and C3, C4 or factor B concentrations in patients with SLE. These results suggest that C4BP is not consumed in these two diseases where strong activation of the classical pathway is known to occur in vivo. In addition, the significantly increased C4BP/C4 ratio, evident in both groups of patients, may provide a protective mechanism against C3 conversion by the classical pathway.