The complement system in bullous pemphigoid. III. Fixation of C1q and C4 by pemphigoid antibody.

By in vitro complement immunofluorescent staining methods, 46 serum samples from bullous pemphigoid patients with titers of antibasement-membrane-zone antibodies of 320 or greater were tested for their ability to fix C1q, C4, and C3. All three staining reactions were positive with 23 of the samples; 21 samples yielded negative reactions. With one sample, the C1q and C3 staining reactions were positive but the C4 reaction was negative; one other sample gave a positive C3 staining reaction only. Absorption of anti-C1q, anti-C4, and anti-C3 with C1q, C4, and C3, respectively, blocked the specific in vitro complement staining. Therefore, bullous pemphigoid antibodies of the IgG type appear to be capable of activating the "classical" complement pathway.