Dynamics of macrophage subsets CD11b^lo (lighter shades) and CD11b^hi (darker shades) in response to aCD24 antibody treatment. WT (red bars) and VCAM-1^−/− blue bars) mice were treated with aCD24 antibody or control immunoglobulin G (see “Methods”). Cellularity (left panels), macrophage (middle panels), and Eb (right panels) responses were followed in the BM (A) and spleen (B). Note the major changes after treatment in the spleen only. These data are consistent with the fact that the spleen and not the BM is the major source of SCF produced after CD24 engagement.⁴⁷  SCF, stem cell factor.