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.

It's possible to have a higher than usual lymphocyte count but have few, if any, symptoms. The higher count usually comes after an illness. It is most often harmless and doesn't last long.

But the higher count might be the result of something more serious, such as a blood cancer or a chronic infection. More tests can show if the lymphocyte count is a cause for worry.

A high lymphocyte count can point to:

• Infection, including bacterial, viral or other type of infection.
• Cancer of the blood or lymphatic system.
• An autoimmune disease causing ongoing, called chronic, swelling and irritation, called inflammation.

Causes of lymphocytosis include:

• Acute lymphocytic leukemia
• Babesiosis
• Brucellosis
• Cat-scratch disease
• Chronic lymphocytic leukemia
• Cytomegalovirus (CMV) infection
• Hepatitis A
• Hepatitis B
• Hepatitis C
• HIV/AIDS
• Hypothyroidism (underactive thyroid)
• Lymphoma
• Mononucleosis
• Severe medical stress, such as from trauma
• Smoking
• Splenectomy
• Syphilis
• Toxoplasmosis
• Tuberculosis
• Whooping cough

A high lymphocyte count is usually found from tests done for other reasons or to help diagnose another condition.

Talk with a member of your health care team about what your test results mean. A high lymphocyte count and results from other tests might show the cause of your illness.

Often, follow-up testing over several weeks shows that the lymphocytosis has cleared up. Special blood tests can be helpful if the lymphocyte count stays high. If the condition stays or the cause isn't known, you might be referred to a doctor who specializes in blood diseases, called a hematologist.