A marked accumulation of eosinophils occurs in several important disorders, such as allergic diseases, parasitic infections, and cancer.¹ The level of eosinophils in the body is normally tightly regulated. In normal subjects, eosinophils account for only a small minority of peripheral-blood leukocytes, and their presence in tissues is primarily limited to the gastrointestinal mucosa.² In certain disease states, however, eosinophils can selectively accumulate in the peripheral blood or any tissue in the body. Any perturbation that results in eosinophilia, defined here as an abnormal accumulation of eosinophils in blood or tissue, can have profound clinical effects. Eosinophilia may be harmful, . . .