IL5 and hypereosinophilic syndrome: In hypereosinophilic syndromes, the overproduction of eosinophils can be attributed to two distinct mechanisms: 1) clonal eosinophil proliferation due to alterations in the hematopoietic lineage and/or receptors involved in eosinophil production; 2) overproduction of eosinophilopoietic cytokines (such as IL-5).