The Eosinophil and Eosinophilia

Because of the eosinophil’s association with unusual disorders, few discoveries on the peripheral blood smear elicit as much trepidation as that of an increased eosinophil count. Despite the ease of eosinophil quantitation afforded by automated leukocyte differential counts, it is important to remember that eosinophils reside mainly in the tissues. In humans, the ratio of bone marrow:blood eosinophils is 3.7–5.1 to 1. In the adult rat, the ratio of tissue:blood eosinophils is about 200–300 to 1. It is here, in the organs themselves, that this cell works both to protect and to damage the host. The eosinophil numbers in the peripheral blood represent an equilibrium among bone marrow production, trafficking in the circulation, and a final destination in tissue, where eosinophil function, apoptosis, and death transpire. The half-life of eosinophils in the circulation is approximately 18 hours, and their total life span is about 6 days.