Biotechnology Hypereosinophilic syndrome (HES) refers to a group of rare blood disorders that are characterized by a marked overproduction of eosinophils (white blood cells that lay an important role in immunity). While the exact prevalence of HES is unknown, the World Health Organization reported age-adjusted incidence of about 3-4 people per 10,000,000 between 2001 and 2005. 28 January 2021