Decrease in surface expression of activation markers on eosinophils 1 month following initiation of imatinib mesylate therapy. The percent of peripheral blood eosinophils expressing CD69 (A), CD25 (B), CD23 (C), and CD40 (D) before treatment and after 1 month of imatinib mesylate therapy is shown for 4 patients with MHES. The shaded boxes indicate the normal ranges for CD69, CD25, and CD40. All of the patient values for percent CD23 are within the normal range (≤ 4.5%). Whereas eosinophil expression of the activation markers CD69, CD25, and CD23 decreased significantly with imatinib mesylate therapy, expression of CD40 was unchanged.