The Lymphocytes test is a common blood test that measures the number of lymphocytes in your blood. Lymphocytes are a type of white blood cell that plays a key role in your immune system. They help your body recognize and fight infections—especially viral infections—and they are involved in making antibodies (proteins that help target germs).

Lymphocytes are usually reported as part of a complete blood count (CBC) with differential, and results may be shown in two ways:

• Absolute lymphocyte count (ALC): the actual number of lymphocytes in a specific volume of blood.
• Lymphocyte percentage: the portion of all white blood cells that are lymphocytes.

In plain language, your results help show whether your immune system is reacting normally. A high lymphocyte count (lymphocytosis) can happen when your body is responding to an infection (often viral) or, less commonly, due to certain immune conditions or blood cancers. A low lymphocyte count (lymphocytopenia) may suggest a weakened immune response, which can occur with some infections, autoimmune diseases, medical treatments (like steroids or chemotherapy), or other health conditions. Because many issues can change lymphocyte levels, doctors interpret the lymphocyte count alongside symptoms and other diagnostic tests.

Doctors order a lymphocyte count to evaluate immune health, investigate signs of infection or inflammation, or follow known medical conditions. It may be included in routine health screening bloodwork or ordered as a targeted part of diagnostic tests when symptoms or other lab results raise concern.

This test is commonly done when a person has symptoms such as:

• Fever, chills, or ongoing fatigue
• Sore throat, swollen glands, or suspected mononucleosis
• Frequent infections or infections that are severe or slow to resolve
• Unexplained weight loss, night sweats, or persistent weakness
• Unusual bruising/bleeding or abnormal results on a CBC

It may also be ordered to help:

• Assess the body’s response to an acute illness (especially viral infections)
• Evaluate possible autoimmune disorders when immune activity may be altered
• Monitor immune status in people taking immunosuppressive medicines, steroids, or undergoing chemotherapy/radiation
• Follow known blood or lymph system conditions and monitor treatment response
• Investigate an enlarged spleen or persistent lymph node swelling

• Viral infections (e.g., influenza, COVID-19, infectious mononucleosis/EBV)
• Chronic viral infections (e.g., hepatitis, HIV infection—often associated with low lymphocytes)
• Whooping cough (pertussis)
• Tuberculosis and other chronic infections
• Autoimmune disorders (e.g., systemic lupus erythematosus, rheumatoid arthritis)
• Primary immunodeficiency disorders
• Medication-related immune suppression (e.g., corticosteroids, chemotherapy, some biologics)
• Leukemia (especially chronic lymphocytic leukemia, CLL)
• Lymphoma (Hodgkin and non-Hodgkin lymphoma)
• Bone marrow disorders or bone marrow suppression

• General wellness and routine health screening as part of a CBC with differential
• Monitoring immune system health over time (especially if you get frequent infections)
• Supporting early evaluation of infection (including common viral infections)
• Tracking immune effects of medications or treatments that may lower white blood cells
• Follow-up monitoring for known blood/lymph system conditions using the lymphocyte count
• Assessing recovery and overall health status after significant illness