Just to throw in my two cents: DR is an activation antigen on T-cells and, as such, is expressed in low and variable amounts on these cells. For this reason, it represents a particular challenge to quantitation by flow cytometry. After activation of T-cells in culture, I believe that there is continuum from low expressing cells to high-ish expressing cells. The number of cells that are said to "express the antigen" therefore depends greatly on: 1) the sensitivity of the cytometer 2) the F/P ratio of the conjugated antibody 3) whether the staining technique is direct or indirect 4) what fluorochrome is being used 5) where you set the "marker" to dichotomize the negative and positive populations (eg at the 1% or 2% or 5% level on the negative control) 6) what you use as the negative control. In addition, depending on whether there has been polyclonal activation or not, it may be that there is a continuous population of dimly expressing cells and that talking about the "% positive" is not as meaningful as it would be if there were a clear intensity distinction between positive and negative cells. On B-cells it is easy --- as they are all positive and all very bright. T-cells are harder. Alice L. Givan Englert Cell Analysis Laboratory Dartmouth Medical School Lebanon, New Hampshire NH 03756 USA tel 603-650-7661 fax 603-650-6130 e-mail givan@dartmouth.edu
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