Why do you use a counterstain with immunohistochemistry on paraffin sections?
Paraffin sections are very thin, and the counterstain allows one to see the tissue morphology and identify the location of any immunolabel that might be present. Without the counterstain, this would be difficult in most cases. The most common counterstain is a blue hematoxylin, although other counterstains (e.g., nuclear fast red) are sometimes used, depending upon the chromagen chosen for visualization of the immunolabel.
