5 Things To Know About Lymphocytes

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Lymphocytes are a type of white blood cell that play a key role in your immune system by helping your body fight infection and disease. Having high lymphocytes can mean you have an infection or disease of your immune system. Having low lymphocytes can mean your immune system is not working as well as it should be and you might be at a higher risk for getting infections.

There are two types of lymphocytes, called T cells and B cells. Each type plays an important role in your immune system.

This article reviews five key facts about lymphocytes, including where they are found, what they look like, how the two types differ, how lymphocyte counts are measured, and how test results are interpreted.

Blood tube sitting on blood results with technician at microscope in lab
Andrew Brookes / Getty Images

Where Are Lymphocytes Found?

Like all blood cells, lymphocytes begin their life’s journey in the bone marrow. This is the spongy, soft tissue located in the center of your bones.

Once lymphocytes are formed, they travel to and perform various functions within the lymphatic system. The lymphatic system is an open network of vessels, organs, and tissues that work together to protect the body from foreign invaders, maintain body fluid levels, and remove cellular waste.

Lymphocytes travel throughout the lymphatic system in a fluid called lymph. Along the network of lymphatic vessels are clusters of small, bean-shaped organs called lymph nodes.

Lymph Nodes and Lymphocytes

Lymph nodes are responsible for filtering lymph so that bacteria, viruses, parasites, fungi, and other invaders can be isolated and neutralized by lymphocytes. Lymph nodes contain the highest concentration of lymphocytes.

Besides your lymph nodes, lymphocytes are also found in lymphoid tissues and organs throughout the body, including your spleen, tonsils, and the lining of your airways.

Types of Lymphocytes

There are two main kinds of lymphocytes, called B cells and T cells. They perform different functions in the immune system to protect your body from infection.

There are also subcategories of B cells and T cells that contribute to both your innate immunity (the generalized immune response you are born with) and your adaptive immunity (the disease-specific immune response that develops whenever you are exposed to a foreign invader).

B Cells

B cell lymphocytes are involved in adaptive, antibody-driven immunity.

Antibodies are proteins produced by the immune system that are specific to each foreign invader your body encounters. They "recognize" the invader by a protein on its surface, called an antigen. Antibody-driven immunity is a tailored response that can more effectively fight infection.

B cells don't kill foreign invaders themselves. Instead, they release antibodies that attach to the antigen of the invader. By doing so, it inactivates the invader and/or "tags" it for destruction by other immune cells.

B cells are produced in the bone marrow, where they mature and specialize into different types of B cells. They then travel to the spleen and lymph nodes to help fight infection.

There are two main types of B cells, each of which has a specific function:

  • Plasma cells produce large volumes of antibodies that target and bind to foreign invaders.
  • Memory B cells help your immune system "remember" foreign invaders so that it can launch a new attack if the invader returns.

T Cells

T cell lymphocytes are mainly involved in adaptive, cell-mediated immunity. This is a type of immunity that does not involve antibodies but instead directly targets and/or kills foreign cells. In essence, it wages cell-to-cell combat.

T cells travel from your bone marrow to a small gland located behind the breastbone called the thymus. It is there that they begin to mature and specialize into different types of T cells.

The different types of T cells have different functions:

  • Cytotoxic T cells directly target and attack foreign invaders like bacteria, viruses, and certain cancer cells.
  • Helper T cells recruit and coordinate other immune cells to help fight infection.
  • Natural killer T (NKT) cells can kill certain tumor cells and also target infected cells for destruction. They function as part of the frontline innate immune response.
  • Regulatory T cells adjust the immune response to avoid overreaction that can cause the body harm (as happens with autoimmune diseases).
  • Memory T cells "remember" foreign invaders so that a new attack can be launched if the invader returns.

What Do Lymphocytes Look Like?

Lymphocytes are not visible to the naked eye. They can be seen when a drop of blood is smeared onto a slide, treated with the right stains, and placed under the lens of a microscope.

When examined under a microscope, lymphocytes will be bigger than red blood cells but fewer in number. Lymphocytes constitute around 20% and 40% of all white blood cells.

Lymphocytes are round and are almost entirely composed of a nucleus (the core of a cell that contains its genetic material in the form of DNA).

With the proper staining, the nucleus of a lymphocyte is dark purple, while the surrounding jelly-like fluid (called the cytoplasm) is a lighter pink.


Blood tests are routinely performed whenever you have signs of an infection and other illnesses. These include tests that measure how many lymphocytes you have in your blood. The tests are easy to perform and only require a simple blood draw, typically from a vein in your arm.

Complete Blood Count

One blood test your healthcare provider may order is a complete blood count with differential (CBC w/ diff). This test is able to reveal the percentage of each type of white blood cell in your blood, including lymphocytes.

The CBC w/ diff can also measure the number of red blood cells (which transport oxygen through the body) and platelets (which are responsible for blood clotting), and the amount of hemoglobin (a protein in red blood cells that carries oxygen molecules) in a sample of blood.

Flow Cytometry

A more advanced test called flow cytometry can identify and count the individual cells in your blood, including lymphocytes.

With flow cytometry, your blood is suspended in a fluid and passed through a laser-generating instrument. The light produced from the laser scatters the cells in such a way that the cells can be individually counted and analyzed.

What Is a Normal Lymphocyte Count?

A normal lymphocyte count varies with age. For healthy adults, the range is between 1,000 and 4,800 cells per microliter of blood (cells/mL). For children, the range is between 3,000 and 9,500 cells/mL.

B and T Cell Screen

A test called the B and T cell screen specifically counts B cells and T cells in a sample of blood. Also known as a B cell or T cell count, the test involves flow cytometry and can provide specific information about how well your immune system is functioning.

The test can also home in on specific types of B cells and T cells. One such example is a CD4 count that measures the number of CD4 T cells that HIV targets for infection. A decrease in the number CD4 T cells in people with HIV is a sign that the disease is progressing.

How to Interpret Test Results

Having too many lymphocytes in your blood is called lymphocytosis, while having too few lymphocytes is called lymphopenia. Both are causes of concern and can arise from infections, diseases, and even certain medical treatments.

Certain diseases can cause both lymphocytosis and lymphocytopenia. For instance, an infection will typically cause lymphocyte counts to increase, but recurrent or severe infections may also cause lymphocyte numbers to plummet.

In other instances, a disease may cause one type of lymphocyte to increase and another type to drop.

Causes of High Lymphocytes

Lymphocytosis is an indication that the immune system has been activated in response to a disease or medical condition. It may be an indication of an infection or a cancer of the blood or lymphatic system, among other things.

Possible causes of lymphocytosis include:

Causes of Low Lymphocytes

Lymphopenia is an indication that lymphocytes are either being depleted or are not being produced in ample quantities. This may be due to autoimmune diseases, infections, or certain medical treatments. Rare genetic disorders may also interfere with the normal production of lymphocytes.

Possible causes of lymphopenia include:


Lymphocytes are a type of white blood cell. They are produced in the bone marrow and move throughout the lymphatic system to help fight infection and diseases. They include B cells that produce antibodies that "tag" foreign invaders for destruction and T cells that directly target and kill foreign invaders.

A normal lymphocyte count varies by your age. Having too many or too few lymphocytes may be a sign of an infection, autoimmune disease, genetic disorder, or cancer. Certain medical treatments can also affect lymphocyte counts.

A Word From Verywell

These days, it is common for people to get their blood test results online before their healthcare providers. If you get your results and see that your lymphocytes are either high or low, don't panic.

On their own, these blood tests only hint at what is going on in your body. They are not diagnostic of any medical condition. A drop or increase may indicate a serious condition, but it could also mean nothing. It is only when the values are compared to other test results that possible causes can be explored.

If you see your lymphocytes are either high or low, ask your healthcare provider why. And, make an effort to understand what the other results mean as well.

Frequently Asked Questions

  • What foods should you avoid if you have a low white blood cell count?

    If your white blood count is abnormally low because your immune system is compromised, you need to avoid foods that may transmit infections. This includes raw milk, yogurts or cheeses made with raw milk, and unpasteurized juices.

  • How common is lymphocytosis?

    Lymphocytosis is very common, especially among people who have had a recent infection. It is not unusual to have a temporary rise in lymphocytes after your body has fought off an infection, particularly a viral one.

  • What are the signs of lymphocytosis?

    Lymphocytosis can lead to lymphadenopathy (swollen lymph nodes) and splenomegaly (an enlarged spleen). Splenomegaly can cause a dull pain in the upper right portion of the body. Other symptoms may develop depending on the underlying cause.

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Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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By Tom Iarocci, MD
Tom Iarocci, MD, is a medical writer with clinical and research experience in hematology and oncology.