Eight cross-linking fixatives were tested for preservation of extracellular or intracellular IgG, IgA, IgM, IgD, kappa and lambda light chains, J chain and secretory component. Most of the selected fixatives have been used in recent immunohistochemical studies of lymphoproliferative processes and comprised routine formalin, glutaraldehyde(1%)-formalin, Baker's formalin-calcium, formalin-sublimate, acetic acid(2%)-formalin-saline, Bouin's fluid, Susa fixative, and carbodiimide. The results obtained in artificial test substrates with defined amounts of IgG or IgA and in biological substrates (colon mucosa, tonsils, and different types of lymphomas) were compared by immunofluorescence with the antigenic preservation afforded by fixation in cold 96% ethanol (with or without inclusion of a pre-fixation 48 h washing period). An antigen concentration at least an eight-fold higher was necessary for detection with most other fixatives. Bouin's and Susa fixatives were peculiar in that they required antigen concentration 150 times higher for detection of IgG but only 3-8 times higher for IgA. Light chains were relatively well preserved by all fixatives except glutaraldehyde. For all cross-linking fixatives, the extent of antigenic masking depended on the concentration of environmental proteins, and the efficiency of unmasking with pronase or trypsin, therefore, varied with the location in the tissue. The J chain was particularly vulnerable to degradation during proteolytic treatment. The extensive masking of extracellular immunoglobulin in formalin-fixed tissue afforded a relatively good signal-to-noise ratio for immunoglobulin-producing cells when kappa and lambda chains were traced. Thus, differentiation between polyclonal and monoclonal B-cell processes on the basis of cytoplasmic labelling was often better in undigested sections. However, the light-chain type of membrane immunoglobulin could usually not be determined in directly fixed tissue. Ethanol fixation preceded by washing in saline afforded such determination and also preserved certain T-cell and HLA-DR antigens as well as diffuse alpha-naphthylbutyrate esterase. Reactive and malignant macrophages could further be traced by their cytoplasmic expression of L1 antigen, both in formalin- and ethanol-fixed material.
