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Human dendritic cells were isolated from tonsils by density gradient separation followed by FACS IV sorting with mAbs to remove contaminating cell populations. The resulting dendritic cell population consisted of large cells with plentiful basophilic cytoplasm, lacking in granules but containing a prominent Golgi apparatus and numerous mitochondria. The cell membrane was irregular, and marked cell protrusions were obvious when stained with anti-HLA class II reagents. Their nuclei were irregular and often indented with a visible nucleolus. These cells were not phagocytic and stimulated autologous and allogeneic lymphocytes more effectively than other tonsil cell types in MLR. Phenotypic analysis of these cells confirmed that they expressed the leucocyte common antigen and stained strongly for HLA- class II antigens. Tonsil dendritic cells also coexpressed the LFA-1 alpha and LFA-1 beta chains but did not stain with a wide variety of anti-monocyte or anti-macrophage antibodies. The cells also lacked Fc and complement receptors and failed to stain with CD1 antibodies. Extensive testing with mAbs revealed only a few positive reactions, and these were consistent with reports of these antibodies staining interdigitating cells in tissue sections. This established that tonsil dendritic cells belong to the unique haemopoietic cell lineage of dendritic cells. No cytoplasmic staining of IL-1 alpha or IL-1 beta was demonstrated, although these lymphokines were readily detected in activated monocytes.