Expression of prion protein versus CD15 by human peripheral blood leukocytes. Leukocytes were isolated from whole blood of normal donors, washed, stained for PrP^C and CD15, and analyzed by flow cytometry. A typical light scatter contour plot of normal peripheral human blood mononuclear cells is shown in the upper left. Analysis gates R1-R3 are set on forward and side scatter alone and define three major subpopulations: lymphocytes (R1), monocytes (R2), and granulocytes (R3). Contour plots of surface prion protein versus CD15 for each of the three subpopulations are shown. Lymphocytes are (PrP⁺, CD15⁻), monocytes are (PrP⁺, CD15^lo-med), and granulocytes are (PrP⁻, CD15^hi). The small amount of PrP⁺ cells in panel R3 are due to contaminating monocytes.