The ultrastructural and cytochemical features of human peripheral blood T(G) cells (T cells with receptors for IgG) and of the cells of the so-called third population (non-T, non-B cells with high avidity receptors for IgG) have been investigated and compared. Both T(G) and third-population cells (TPC) contained acid hydrolases with a paranuclear localization of α-naphthyl acid esterase, β-glucuronidase or acid phosphatase. At the electron microscopy level, T(G) and TPC were indistinguishable and displayed rough cell surface, indented nuclei, abundant cytoplasm with predominance of the smooth over the rough membranes and peroxidase-negative granules. A large proportion of cells of the TPC could form rosettes with sheep erythrocytes after treatment with neuraminidase. The observed close similarities between T(G) and TPC may suggest that both cell types belong to a special subset of T cells. However, the alternative hypothesis that both T(G) and TPC are part of a subset unrelated to T cells, such as a new non-T, non-B cell population, or even of the monocytic-macrophage lineage, is also discussed.
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