Mature splenic (top) and progenitor (bottom) DCs express high levels of B7-1 and B7-2 on their surface. In vitro–activated DCs were, upon harvesting, treated with 2.4G2 antibodies to block Fc receptors and subsequently incubated with FITC-labeled B7-1 or B7-2 antibodies. The cells were analyzed by FACS and their fluorescence signals are represented in overlay histograms.