The effect of IL‐4 on antigen expression: A comparison of fluorescence intensity and antigen density

We have compared the membrane expression of MHC class II antigen and Ig on murine splenic B lymphocytes by two flow cytometric methods. The first method, commonly used, determined the mean immunofluorescence intensity; the second measured antigen density as a ratio of mean fluorescence intensity to cell volume. The data generated by the two methods allow for different interpretations of antigen density in comparison lo antigen expression. By mean fluorescence intensity, activated B lymphocytes expressed more MHC class II antigen and membrane Ig than did resting B cells. However, resting B cells had the same density of MHC class II antigen and membrane Ig as activated B cells. Next we examined the effect of IL‐4 on antigen expression by the two methods. We found that although lL‐4 increased the mean fluorescence intensity and density of MHC class II and membrane Ig on each B cell population, the density of expression of membrane Ig tended to be greater on the resting B cells than on the activated, while the density of MHC class II antigen was essentially the same on each of the IL‐4 treated B cell populations.