The Eosinophil (EOS) test measures the count of eosinophils, which are microscopic white blood cells produced in the bone marrow that help the body fight conditions such as parasitic infections, allergic reactions, or certain types of cancer. Eosinophils can be found in various body tissues, including blood, bone marrow, skin, adipose tissue, lungs, and the stomach. Performed as part of a complete blood count (CBC), this test determines the percentage of eosinophils in the blood. Abnormalities in eosinophil levels can generally indicate the presence of an infection, an allergic reaction, or another health issue in the body.