Lymphocytosis is an abnormal increase in the number of lymphocytes, a type of white blood cell. White blood cells help fight infection. Lymphocytosis isn't a disease but a sign of an underlying problem. It causes no symptoms and may be found incidentally on routine blood tests done for some other reason. A significant increase in lymphocytes may indicate a more serious underlying illness, such as chronic lymphocytic leukemia. Treatment is directed at the underlying cause of lymphocytosis.