Eosinophils are produced in the bone marrow (BM) and differentiate from myeloid progenitors in response to IL‐3, GM‐CSF, and more selectively, IL‐5.1, 2 Although eosinophils are normally a rare population, upon IL‐5 stimulation, which can occur in response to a parasitic infection and inflammatory disease, the BM can produce several orders of magnitude more eosinophils than are seen at steady state. This evidence concerns the gene IL5 and parasitic infectious disease.