C3d immunohistochemistry on formalin-fixed tissue is a valuable tool in the diagnosis of bullous pemphigoid of the skin

Background: Direct immunofluorescence (DIF) testing is an important procedure in the diagnosis of autoimmune bullous dermatoses. We investigated the expression of C3d in formalin-fixed, paraffin-embedded tissue of autoimmune bullous dermatoses. Methods: The immunohistochemical expression of C3d in bullous pemphigoid (BP) (n = 32), pemphigoid gestationis (PG) (n = 3), pemphigus (n = 14), dermatitis herpetiformis Duhring (DHD) (n = 10), linear immunoglobulin A (IgA) dermatosis (n = 4), mixed forms of BP and linear IgA dermatosis (n = 2), and 44 controls was analyzed on formalin-fixed tissue. Results: Thirty-one of 32 cases (97%) of BP and 3 out of 3 cases (100%) of PG showed a linear positivity of C3d along the basement membrane. Only 3 out of 14 (21%) cases of pemphigus showed an intraepidermal intercellular expression of C3d. The two mixed forms of linear IgA dermatosis and BP showed a linear positivity of C3d along the basement membrane. All cases of DHD, linear IgA dermatosis and all of the controls were negative for C3d. Conclusions: C3d immunohistochemistry is a valuable tool in the diagnosis of BP and PG of the skin with a sensitivity of at least 97%. Mixed forms of linear IgA dermatosis, and BP, DHD and linear IgA dermatosis can only be identified by DIF. A positive result may prompt serologic confirmation of BP without further need for DIF. Pfaltz K, Mertz K, Rose C, Scheidegger P, Pfaltz M, Kempf W. C3d immunohistochemistry on formalin-fixed tissue is a valuable tool in the diagnosis of bullous pemphigoid of the skin.