An immunohistological study of reactive lymphoid tissue

The aim of this study was to document the patterns of cytoplasmic Ig heavy and light chain expression in reactive lymphoid tissue, using single and double immunoenzymatic labelling techniques. This investigation was undertaken, firstly, to provide information on whether the normal counterparts of high grade lymphoma cells (e.g. centroblasts, immunoblasts) ever express more than one light or heavy chain (as has been noted in the past for lymphomas) and also, secondly, to seek evidence of intraclonal ‘switching’ from cytoplasmic IgM to cytoplasmic IgG expression. Paraffin embedded sections, all showing substantial reactive changes, were analysed by means of immunoperoxidase stains for the three major immunoglobulin classes (IgG, IgM and IgA), both light chain classes and J chain. In addition, double immunoenzymatic labelling techniques were used to search for cells showing simultaneous expression of kappa and lambda light chains and cells expressing mu and gamma heavy chain. Large transformed lymphocytes showing cytoplasmic Ig‐staining in the pulp and interfollicular areas often have nuclear morphology indistinguishable from germinal centre centroblasts. There was no evidence of primitive appearing IgM‐positive cells and IgG‐positive cells of more mature morphology. In addition, immunoenzymatic staining showed that cells simultaneously expressing both IgG and IgM are only rarely encountered. When such cells were detected, the morphology was not that of a blast cell, but rather of a plasma cell containing Russell bodies. Hence it is suggested that cytoplasmic IgM switching to IgG is rarely detected by immunohistological methods in reactive tissue. Double staining for kappa and lambda revealed that cells simultaneously expressing both light chain types were not detected even among cells showing the most primitive morphology.