The absolute Lymphocyte Count test is performed on a sample of blood to measure the level of Absolute Lymphocyte Count in the blood. Lymphocytes are one of the several different types of white blood cells. White blood cells are an important part of your immune system. They help your body fight antigens, like bacteria, viruses and other toxins that make you sick. If you have a weak immune system, it means that there are not enough white cells in your bloodstream. The test is performed to detect Infectious Mononucleosis, Leukemia, Lymphoma, and tuberculosis.