An original perifollicular zone cell in the canine reactive lymph node: a morphological, phenotypical and aetiological study.

In this study of 109 canine reactive lymph nodes, the perifollicular zone (PZ) cell was characterized by cytological, histological, immunocytochemical and electron microscopical techniques. The PZ cell was always found in association with plasma cell hyperplasia. Its main cytological characteristics were medium size, fine chromatin and a large central prominent nucleolus with a small amount of pale cytoplasm. It was located in a clearly recognizable PZ surrounding the follicles; this zone was particularly well developed at the capsular pole of the lymph node. Electron microscopical findings indicated a poorly differentiated cell. Immunolabelling indicated a CD3-, cIg-, Ki-67- immunophenotype, suggesting a resting B cell. These results suggest that the PZ cell belongs either to a post-follicular stage between large immunoblasts and plasma cells or, as is more likely, to a pre-follicular lymphoid subpopulation occurring early in the B-cell differentiation scheme, as with most human marginal zone (MZ) cells. Its high frequency of occurrence in reactive lymph nodes in mammary tumour lymphadenopathies, systemic lupus erythematosus and leishmaniasis, suggests that the PZ cell has a special role in the canine immune response, or perhaps in the arrested maturation of the normal developmental process.