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Lymphocytes are a type of white blood cell that play a crucial role in your body's immune system. They help protect you against infections, viruses, and other harmful substances. There are three main types of lymphocytes: B cells, which produce antibodies to fight bacteria and toxins; T cells, which destroy infected or cancerous cells; and natural killer cells, which attack virus-infected cells and tumor cells. A lymphocyte count is typically measured as part of a complete blood count, which is a common laboratory test that evaluates the different components of your blood.

The normal range for lymphocytes in adults is generally between 1,000 and 4,800 lymphocytes per microliter of blood, though this can vary slightly depending on the laboratory. In children, the normal range tends to be higher. When your lymphocyte count falls outside the normal range, it may indicate an underlying health condition. A high lymphocyte count, called lymphocytosis, can occur due to viral infections such as mononucleosis, bacterial infections like tuberculosis or whooping cough, or certain blood cancers including leukemia and lymphoma. A low lymphocyte count, known as lymphocytopenia, may result from autoimmune disorders, certain medications, chemotherapy or radiation therapy, HIV/AIDS, or inherited immune deficiency disorders.

Your doctor may order a lymphocyte count if you have symptoms of an infection, such as fever, fatigue, or swollen lymph nodes, or if they suspect an immune system disorder or blood cancer. The test requires a simple blood sample, usually drawn from a vein in your arm. No special preparation is typically needed, though you should inform your healthcare provider about any medications or supplements you are taking, as some can affect your results.

If your lymphocyte count is abnormal, your doctor will consider your symptoms, medical history, and other test results to determine the cause. Additional tests may be needed to pinpoint the specific condition affecting your lymphocyte levels. Treatment depends entirely on the underlying cause and may range from simply monitoring your levels over time to treating an infection or managing a chronic condition. Regular monitoring of lymphocyte counts can be important for people with known immune system disorders or those undergoing certain medical treatments.