Lymphocytosis (lim-foe-sie-TOE-sis), or a high lymphocyte count, is an increase in white blood cells called lymphocytes (LIM-foe-sites). Lymphocytes are an important part of the immune system. They help fight off diseases, so it's normal to see a temporary rise in the number of lymphocytes after an infection.

A count significantly higher than 3,000 lymphocytes in a microliter of blood is generally considered to be lymphocytosis in adults. In children, the threshold for lymphocytosis varies with age, but may be as high as 7,000 to 9,000 lymphocytes per microliter. The exact thresholds for lymphocytosis may vary slightly from one lab to another.

A high lymphocyte count is usually found when your doctor has ordered tests to help diagnose a condition you're already experiencing. It's rarely an unexpected finding or simply discovered by chance. Talk to your doctor about what your test results mean. A high lymphocyte count and results from other tests may already indicate the cause of your illness, or your doctor may suggest other tests to further check your condition.

Jul. 30, 2013