HES is a primary disorder of eosinophils due to clonal overproduction in the bone marrow. HES is characterized by a persistent increased eosinophil count (>1.5 × 109/L) and eosinophil-related tissue damage. There are three categories of HES including: myeloproliferative, idiopathic, and a novel HES variant called "lymphocytic" HES. Eosinophilia that is associated with allergic reactions does not indicate the presence of HES.
Note the eosinophilia present in the peripheral blood of this Hypereosinophilic Syndrome patient.