Detection of OKT6-positive cells (without visible Birbeck granules) in normal peripheral blood.

The specificity of a monoclonal antibody (OKT6) for peripheral blood mononuclear cells was examined by indirect immunofluorescence and ultrastructural immunogold labelling. Some rare peripheral blood mononuclear cells (approximately 1%) expressed T6 antigen on their membrane surface, as determined by light microscopy and cytofluorometry. Electron microscopic examination of immunogold-labelled cells revealed that OKT6-positive cells were dendritic, lacking the Birbeck granules and expressed variable density of the membrane T6 antigen. The relationship of such cells with Langerhans' cells is discussed.