Definition of lymphocytosis
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.
Causes of lymphocytosis
You may have a lymphocyte count that is higher than would normally be expected but have few, if any, symptoms. Your doctor must then decide if this is a harmless, temporary situation, as can occur after an illness, or if it represents something more serious, such as a blood cancer or a chronic infection. Before deciding if a lymphocyte count is “too high” or is a cause of concern, your doctor may need to perform other tests.
If your doctor determines your lymphocyte count is high, the test result may be evidence of one of the following conditions:
- Infection (bacterial, viral, other)
- Cancer of the blood or lymphatic system
- An autoimmune disorder causing ongoing (chronic) inflammation
Specific causes of lymphocytosis include:
- Acute lymphocytic leukemia
- Chronic lymphocytic leukemia
- Cytomegalovirus (CMV) infection
- Multiple myeloma
- Other viral infections
- Whooping cough
When to see a doctor
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.