DIF has remained the method of diagnosis of autoimmune blistering skin diseases because there are no other techniques that can match the sensitivity and specificity. Secondary antibodies are available from Sigma. There has a been a paper by Pfalz et al. (2009) showing that C3d immunohistochemistry is a valuable tool in the diagnosis of BP of the skin and had sensitivity of  97%. However limitations were that linear IgA dermatitis, and DH, could only be identified by DIF and hence DIF still remains the gold standard.

Reference:

PfaltzK,Mertz K, Rose C, Scheidegger P, Pfaltz M, KempfW.C3d

immunohistochemistry on formalin-fixed tissue is a valuable tool in the

diagnosis of bullous pemphigoid of the skin.

J Cutan Pathol 2009.

http://www.allergieundhaut.ch/wp-content/uploads/2013/01/2009_10_15_Bulloeses_Pemphigoid.pdf