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.
June 15, 2016
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- Kaushansky K, et al. Lymphocytosis and lymphocytopenia. In: Williams Hematology. 9th ed. New York, N.Y.: The McGraw-Hill Education; 2016. http://accessmedicine.com/book.aspx?bookID=1581. Accessed May 19, 2016.
- AskMayoExpert. Lymphocytosis? Rochester, Minn.: Mayo Foundation for Medical Education and Research; 2015.