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.

If your doctor determines your lymphocyte count is high, the test result may be evidence of one of the following conditions:

Specific causes of lymphocytosis include:

1. Acute lymphocytic leukemia
2. Chronic lymphocytic leukemia
3. Cytomegalovirus (CMV) infection
4. HIV/AIDS
5. Mononucleosis
6. Multiple myeloma
7. Other viral infections
8. Tuberculosis
9. Vasculitis
10. Whooping cough

Causes shown here are commonly associated with this symptom. Work with your doctor or other health care professional for an accurate diagnosis.

Source: mayoclinic.org