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Lymphocytosis (high lymphocyte count)


Lymphocytosis (lim-foe-sie-TOE-sis), or a high lymphocyte count, is an increase in white blood cells called lymphocytes. 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. It may be as high as 9,000 lymphocytes per microliter. The exact thresholds for lymphocytosis may vary slightly from one lab to another.


You may have a lymphocyte count that is higher than would normally be expected but have few, if any, symptoms. It's usually a harmless, temporary situation, as can occur after an illness. But it may represent something more serious, such as a blood cancer or a chronic infection. Your doctor may need to perform other tests to determine if your lymphocyte count is a cause for concern.

If your doctor determines that 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
  • Mononucleosis
  • Other viral infections
  • Tuberculosis
  • Vasculitis (blood vessel inflammation)
  • 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 with your doctor about what your test results mean. A high lymphocyte count and results from other tests may indicate the cause of your illness. Or your doctor may suggest other tests to further check your condition.

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