Lymphocytosis (lim-foe-sie-TOE-sis), also known as a high lymphocyte count, is an increase in white blood cells called lymphocytes. Lymphocytes help fight off diseases. It's typical for the lymphocyte count to rise briefly after an infection.

A count much higher than 3,000 lymphocytes in a microliter of blood defines lymphocytosis in adults. In children, the number of lymphocytes for lymphocytosis varies with age. It can be as high as 8,000 lymphocytes per microliter. The numbers for lymphocytosis can differ some from one lab to another.