Bone marrow findings in a representative HES patient before and at 1 month following the initiation of imatinib mesylate therapy. (A-C) Before therapy. (D-F) After therapy. Sections were stained with hematoxylin and eosin (A,D), antireticulin antibody (B,E), and antitryptase antibody (C,F). Not only was there a dramatic decrease in eosinophils and myeloid precursors in response to imatinib mesylate (A,D), but a resolution in the hypercellularity (A,D), reticulin fibrosis (B,E), and atypical spindle-shaped mast cells (C,F) evident prior to therapy.