Lymphocytes: are a type of white blood cell that help your body fight viruses and support long-term immunity. They are important for recognizing and remembering germs your body has seen before. Normal lymphocyte levels indicate a healthy immune system. Low lymphocytes (lymphopenia) may make it harder for your body to fight infections and can be caused by certain illnesses, medications, or immune conditions. High lymphocytes (lymphocytosis) often occur with viral infections, certain chronic conditions, or immune system activation.