Eosinophils are components of white blood cells and, like other granulocytes, are derived from hematopoietic stem cells in the bone marrow. Eosinophils have a role in killing bacteria and parasites and are extremely important cells in the process of immune and allergic reactions. Eosinophils can release granular contents that cause tissue damage and promote inflammation. The number of eosinophils in the blood fluctuates periodically, with a decrease in the number of cells in the morning and an increase at midnight. When blood glucocorticoid concentrations are elevated, the number of eosinophils decreases, and when glucocorticoid concentrations decrease, the number of eosinophils increases. In addition, the percentage of eosinophils in normal people ranges from 0.5% to 5%. If the eosinophils in the body are not at this level, it is called abnormal. For people with elevated or decreased eosinophils, it is recommended to go to the hospital for examination to identify whether it is physiological or pathological. If the test is abnormal, treatment can be carried out according to the doctor’s instructions.