Paraformaldehyde is used in immunohistochemical (IHC) applications as a fixative. It's reactivity causes a covalent modification of the proteins in the sample. This is rarely a problem when using polyclonal antibodies but monoclonal antibodies can be adversely affected. It is possible for the PFA modification to be in the region of antibody binding.
The
symbol indicates the antibody is adversely affected by the use of PFA. An alternative fixation protocol is advised.