Flow cytometric analysis of CR1 and MHC class I on resting and activated RBL-CR1 shows that the increased cell surface expression of CR1 on activated cells coincides with decreased expression of MHC class I. Cells were either incubated in HBSS/g (resting) or in HBBS/g containing 1 μmol/L A23187 and 1.2 mmol/L CaCl₂ (activated) for 1 hour at 37°C. Cells were stained using anti-MHC class I, W6/32 (lower panels), or anti-CR1, 3D9 (upper panels) as described, and analyzed by FACS analysis. Light lines are for isotype-matched control as first antibody.