Eosinophils are white blood cells that participate in the immune response, states MedlinePlus. The absolute eosinophil count measures the number of eosinophils present in a blood sample. An absolute eosinophil count of less than 350 cells per microliter is considered normal.
Some diseases, such as asthma and leukemia, cause eosinophils to be more active than usual. If a person has an elevated eosinophil count, it may also be due to hay fever, eczema or an autoimmune disease. An elevated eosinophil count is also called eosinophilia, reports MedlinePlus.
A blood differential measures the percentage of eosinophils in the blood, according to MedlinePlus. This test also measures the percentages of neutrophils, monocytes, basophils and lymphocytes in the blood. Eosinophils should make up only 1 to 4 percent of the total white blood cell count. A higher-than-normal percentage of eosinophils may indicate the presence of cancer, parasitic infection, collagen vascular disease or Addison's disease. The percentage of eosinophils in the blood also increases when someone has an allergic reaction.
Eosinophils are responsible for destroying cancer cells and attacking parasites, explains the University of Rochester Medical Center. Eosinophils also play a role in responding to allergens. Infections, cancer and a weakened immune system all have the potential to affect the number of eosinophils in the blood.