A high lymphocyte count, known as lymphocytosis, may indicate nothing at all or may indicate a condition such as cancer, explains Mayo Clinic. Often, lymphocytes in the blood are higher after an illness, which is a temporary, harmless situation.
If a doctor determines a person has high lymphocytes, it may mean the person has an infection, has cancer in the lymphatic system or blood or has an autoimmune disorder, according to Mayo Clinic. Some of the specific conditions that may cause high lymphocytes include leukemia, mononucleosis, multiple myeloma, tuberculosis or whooping cough. Depending on other symptoms, a doctor may perform tests before making a diagnosis. Since there are usually no signs a person has a high number of lymphocytes, the observation is typically made during a blood test for another condition.
Lymphocytes are white blood cells that fight disease in the body, so it is normal to see a temporary rise in their number following an illness, reports Mayo Clinic. Any count above 3,000 lymphocytes per microliter of blood is considered high in adults. In children, the number of lymphocytes in the blood may vary with age, but usually any number over 7,000 to 9,000 lymphocytes per microliter is high. The exact number of lymphocytes per microliter may vary slightly from one lab test to another.