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