Non-specific labelling of mast cells in feline oral mucosa--a potential problem in immunohistochemical studies.

Non-specific labelling of mast cells was found to occur in formalin-fixed sections of feline oral mucosa during immunohistochemical procedures. The phenomenon occurred when normal goat, rabbit or mouse serum was applied as a negative control in place of primary antibodies. In addition, with murine isotype-specific negative control reagents, non-specific labelling of mast cells was intense when IgG2b was applied as the primary reagent, but absent or mild when IgGl or IgG2a isotypes were utilized. The non-specific labelling could be eliminated or diminished by reducing the pH of the washing and dilution buffers from 7.4 to 6.0, or by preincubating sections with heparin. The non-specific binding could also be abolished by preincubating sections with heparinase-I. The results suggest that the non-specific binding was mediated by heparin present within feline oral mucosal mast cells. These findings illustrate the importance of the inclusion of adequate control sections in immunohistochemical studies.