What are lymphocytes?

Lymphocytes are a type of white blood cell. They play an important role in your immune system, which helps your body fight disease and infection. Your immune system is made up of an intricate web of immune cells, lymph nodes, lymph tissue and lymphatic organs. Lymphocytes are a type of immune cell. There are two main types of lymphocytes:

  • T lymphocytes (T cells): T cells control your body’s immune system response and directly attack and kill infected cells and tumor cells.
  • B lymphocytes (B cells): B cells make antibodies. Antibodies are proteins that target viruses, bacteria and other foreign invaders.
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What do lymphocytes do?

Lymphocytes help your body’s immune system fight cancer and foreign viruses and bacteria (antigens). Lymphocytes help your immune system remember every antigen it comes in contact with. After an encounter, some lymphocytes turn into memory cells. When memory cells run into an antigen again, they recognize it and quickly respond. This is why you don’t get infections like measles or chickenpox more than once. It’s also the reason getting vaccinated can prevent certain diseases.

How do T cells and B cells work?

Your T cells and B cells work together. They each have different roles in your immune system.

Your T cells help kill infected cells and control your body’s immune response to foreign substances. Most of your T cells need the help of another immune cell to become activated. After your T cells are activated, they multiply and specialize into different types of T cells. These types include:

  • Cytotoxic (killer) T cells: Cytotoxic T cells attach to antigens on infected or abnormal cells. Then, they kill the infected cells by making holes in their cell membranes and inserting enzymes into the cells.
  • Helper T cells: Helper T cells help your other immune cells. Some helper T cells help B cells make antibodies against foreign invaders. Others help activate cytotoxic T cells.
  • Regulatory (suppressor) T cells: Regulatory T cells make substances that help end your immune system’s response to an attack. Sometimes, they prevent harmful responses from occurring.

B cells have receptors on their surfaces where antigens attach. B cells learn to recognize the different antigens and produce specific antibodies to attack each one. The B cells respond to antigens in two ways:

  • Primary immune response: When an antigen attaches to a receptor, your B cells are stimulated. Some B cells change into memory cells. Other B cells change into plasma cells. Plasma cells make an antibody specific to the particular antigen that stimulated it. Production of enough of that specific antibody can take several days.
  • Secondary immune response: If your B cells encounter that antigen again, the memory cells remember it and multiply. They change into plasma cells and quickly produce the correct antibody.


Where are lymphocytes located?

Lymphocytes develop in your bone marrow. Then, they mature and exit into your bloodstream. Mature lymphocytes are found in your blood and all parts of your lymphatic system. Some lymphocytes travel to your thymus gland. These lymphocytes become T cells. Other lymphocytes travel to your lymph nodes and organs. These lymphocytes become B cells.

What do lymphocytes look like?

Lymphocytes are bigger than red blood cells, but they’re still microscopic. Each tiny lymphocyte has a large nucleus at its center. The nucleus is dark purple. The surrounding jelly-like fluid (cytoplasm) is purplish.

Conditions and Disorders

What is the normal range of lymphocytes?

Lymphocyte levels vary depending on your age, race, sex, altitude and lifestyle.

In adults, the normal range of lymphocytes is between 1,000 and 4,800 lymphocytes in every 1 microliter of blood. In children, the normal range of lymphocytes is between 3,000 and 9,500 lymphocytes in every 1 microliter of blood. About 20% to 40% of your white blood cells are lymphocytes.

What does a high level of lymphocytes mean?

High levels of lymphocytes in your blood are called lymphocytosis. Lymphocytosis is usually due to an infection or illness. Your body sometimes produces extra lymphocytes to help fight infections and illnesses. But a more serious condition can also cause a high lymphocyte count, including:

  • Hepatitis.
  • Syphilis.
  • Mononucleosis.
  • Tuberculosis.
  • HIV and AIDS.
  • Underactive thyroid (hypothyroidism).
  • Infections such as whooping cough, toxoplasmosis or cytomegalovirus.
  • Blood cancers such as lymphoma or leukemia.

What does a low level of lymphocytes mean?

Low levels of lymphocytes in your blood are called lymphocytopenia (or lymphopenia). The flu or other mild infections can cause lymphocytopenia, but it could also be caused by a more serious disease or condition, including:

  • HIV or AIDs.
  • Tuberculosis or typhoid fever.
  • Viral hepatitis.
  • Blood diseases such as Hodgkin’s disease
  • Autoimmune diseases such as lupus.
  • Rare inherited conditions such as severe combined immunodeficiency (SCID), ataxia-telangiectasia, DiGeorge syndrome and Wiskott-Aldrich syndrome.
  • Radiation or chemotherapy treatments.

What are the common tests to check the health of my lymphocytes?

There are a couple of blood tests that can tell you the number of lymphocytes in your blood. These tests include:

  • Absolute lymphocytes count: The absolute lymphocytes count is part of a complete blood count (CBC). An absolute lymphocytes count tells you the number of cells as an absolute number instead of as a percentage. Multiply your total number of white blood cells by the percentage of your white blood cells that are lymphocytes to get your absolute lymphocytes count.
  • Flow cytometry: With flow cytometry, your blood is processed in a special laboratory. A technician suspends your blood in a fluid and sends it through a laser instrument called a flow cytometer. The laser’s light scatters your blood cells into patterns that make the different types of cells easy to see and count.

What are common symptoms of lymphocyte conditions?

Lymphocytosis and lymphocytopenia alone usually don’t cause any symptoms. But if a blood disorder or cancer causes a high lymphocyte count, you may have symptoms associated with the disease. These symptoms may include:

A note from Cleveland Clinic

Lymphocytes are a type of white blood cell. They’re a part of your immune system, which helps your body fight disease and infection. Your lymphocyte count can be taken during a normal blood test at your healthcare provider’s office. If your lymphocyte count is higher or lower than average, you may be worried about what that means. Many times, a high or low number means your body has been helping you fight an infection. If you’re concerned, reach out to your healthcare provider so they can determine the underlying cause and get you back to normal.

Last reviewed by a Cleveland Clinic medical professional on 06/20/2022.


  • American Society of Hematology. Blood Basics. ( Accessed 6/20/2022.
  • Merck Manual. Acquired Immunity. ( Accessed 6/20/2022.
  • National Human Genome Research Institute. Lymphocyte. ( Accessed 6/20/2022.

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