Lymphocytes normally represent 20% to 40% of circulating white blood cells. When the percentage of lymphocytes exceeds 40%, it is recognized as relative lymphocytosis.
Causes of lymphocytosis include: Acute viral infections, such as infectious mononucleosis (glandular fever), hepatitis and Cytomegalovirus infection. Other acute infections such as pertussis.
A number of conditions can cause low lymphocyte count: Blood diseases like Hodgkin's disease. Autoimmune disorders like lupus. Infectious diseases like tuberculosis, AIDS, and viral hepatitis.