It can not be a rabbit and I use citrate buffer pH 6 + heat for antigen retrievel, which works fine for the other antibodies. I have tried rat anti CD18, both clone M18/2 and C71/16, without any success. When looking around now I saw that BD Bioscience say that CD45 clone 30-F11 works in IHC-P. We always use that for Flowcytometry and the CD18 for frozen sections, so I didn´t think about CD45 at all. Have somebody tried CD45 clone 30-F11 on paraffin sections? Other suggestions?