HES can directly affect the eosinophil lineage (often linked to a fusion gene FIP1L1-PDGFRA, and corresponding in this case to chronic eosinophilic leukemia), or the lymphoid lineage, where eosinophilia is secondary to expansion of a T cell subset overproducing interleukin-5, a cytokine involved in eosinophilopoiesis. 