Lymphocytes are one type of white blood cells that help the body fight off infection, so they are always present in blood, according to WebMD. However, lymphocytes should only make up 25 to 40 percent of all white blood cells. An abnormal lymphocyte count means a blood test detected an irregular increase or decrease in the ratio of lymphocytes to total white blood cell count. Doctors use these results to monitor diseases and infections, such as leukemia.
Lymphocytosis occurs when blood develops a high white blood cell count, explains Mayo Clinic. Lymphocytes often rise temporarily when the body is exposed to infection, but in general, blood shouldn’t contain more than 3,000 lymphocytes per microliter in adults. Depending on a child’s age, lymphocytosis is considered a high count of 7,000 to 9,000 lymphocytes in a microliter of blood. Lymphocytosis may not cause any symptoms, and since it is not a definitive sign of disease, doctors usually perform further tests to diagnose other conditions, including cancer, mononucleosis, HIV and tuberculosis.
Lymphocytopenia, also known as lymphopenia, occurs when lymphocytes per microliter are lower than 1,000 in adults and 3,000 in children, notes the National Heart, Lung, and Blood Institute. A harmful drop in lymphocytes makes the body vulnerable to infection. Lymphocytopenia is typically caused by an immune system problem that hinders lymphocyte production or when high amounts of white blood cells are trapped or destroyed.