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SUMMARYIn r(7rf> experiments have documented the role of cytokines in the regulation of ihc human humoral immune response. Which cytokines are operative in vivo and in which lymphoid compartment interactions between cytokinc-producing T cells and antibody-forming B cells occur is still unclear. For that reason we studied human tonsils using immunohislochemical techniques. In tissue sections from tonsils in a resting stage after recurrent tonsillitis we observed cells producing IL-1 a and tumour neerosis factor-alpha (TNF-a) whieh were exclusively localized in the mantle zone of the follicle and in the exlrafollicular area. Furthermore, a high frequency oi' interferon-gamma (IFN--/)-producing cells was delected in the extrafollicular area, but nol inside the follicles. Occasional IL-2-and IL-4-producing cells were found in ihc exirafollieular area, Immunohistochemieal detection of antibody isoiypes revealed that B cells, IgM-membrane-positive. were localized inside the follicles and manlle zones, whereas igD-membrane-positive cells were mainly found in the mantle zones of secondary follicles-In contrast, plasma cells producing IgG I 4 and IgA 1-2 were found in the extrafollicular area. No IgD and IgE antibody-forming cells were delected in tonsils, whereas IgM antibodyforming eells were detected in the extrafollicular area. The co-localization of cytokine-producing cells and antibody-forming cells in human tonsil suggests thai T B ceil interactions, required for B cell differentiation and isotype switching, take place in the extrafollicular area.Keywords cytokine-producing cells antibody-forming cells palatine tonsils isotype switching in situ human histochemistry

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