Eosinophils are a type of white blood cell. The eosinophil ratio, which is the percentage of eosinophils in white blood cells, is normal at 0.5%-5%. The red blood that flows through the blood vessels is made up of a non-cellular component, also called plasma, which is liquid; and a cellular component called blood cells, which are suspended in the plasma and flow with the blood. Blood cells are subdivided into red blood cells, white blood cells and platelets. White blood cells include five types of cells, two types of granulocytes and three types of granulocytes. The granulocytes include lymphocytes and monocytes; the three granulocytes include neutrophils, eosinophils, and basophils. Of the five types of cells, neutrophils are the most numerous and normally make up 50-70% of the total number of white blood cells, followed by lymphocytes, which make up 20-40% of the total; the other three types of cells make up a smaller percentage. Eosinophils are mainly involved in the body’s anti-allergic and anti-parasitic reactions, so the percentage of eosinophils increases in allergic and parasitic diseases. If the percentage of eosinophils decreases, it is of little clinical significance.