EOS (Eosinophils): a type of white blood cell that help your body fight allergies, parasites, and certain infections. They are part of your immune system’s response to irritants and foreign invaders. Normal eosinophil levels indicate your immune system is balanced. High eosinophils (eosinophilia) may occur with allergic reactions, asthma, parasitic infections, or certain autoimmune conditions. Low eosinophils are usually not a concern.