Eosinophilia refers to an unusually high count of eosinophils in the bloodstream. Typically, eosinophils make up 1 to 3% of peripheral blood, with a normal range of 350 to 650 eosinophils per cubic millimeter. This condition can be classified into three levels: mild (fewer than 1500 eosinophils per cubic millimeter), moderate (1500 to 5000 per cubic millimeter), and severe (over 5000 per cubic millimeter). In regions where certain diseases are prevalent, these illnesses are often the primary cause of eosinophilia. In more developed countries, eosinophilia is usually attributed to allergies or, less frequently, reactions to medication. Although there are many other potential causes, they are rare on an individual basis. Eosinophilia can be either primary or secondary. Primary eosinophilia arises from an abnormality in a hematopoietic stem cell, such as in eosinophilic disorders. Secondary eosinophilia results from a reactive process stimulated by cytokines, as seen in allergic reactions.
