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.