Eosinophils were first described in 1879 by Paul Ehrlich, based on their staining behavior. Eosinophils are a subpopulation of leukocytes, originating in the bone marrow from a CD34+ precursor. They circulate in peripheral blood, where they represent 2% to 3% of total white cells, and have a half-life of approximately 8 to 18 hours. Nevertheless, once recruited in peripheral tissues by specific cytokines, eosinophils can survive for days or weeks as resident cells. Eosinophils possess a powerful enzymatic machinery (eosinophil-derived neurotoxin, eosinophil cationic protein, and major basic protein) not shared with other types of granulocytes.1