High lymphocyte count (lymphocytosis): What causes it?

What causes lymphocytosis?

- Dawn / Florida

Answer

Lymphocytosis is an abnormal increase in the number of lymphocytes — a type of white blood cell — in your blood. White blood cells help fight infection.

A normal, healthy adult has an absolute lymphocyte count (ALC) of 1,300 to 4,000 per microliter of blood. An ALC over 4,000 indicates lymphocytosis.

Lymphocytosis isn't a disease but a sign of an underlying problem. Common causes include viral and bacterial infections, such as mononucleosis, influenza or whooping cough (pertussis). A marked increase in lymphocytes may indicate a more serious condition, such as chronic lymphocytic leukemia.

Lymphocytosis typically causes no symptoms and is often discovered incidentally when a blood test is done for some other reason.