Lymphocytes are a type of white blood cell. They play a key role in your immune system by helping your body fight infection and disease.
High or low lymphocyte levels can indicate a problem with your immune system. Some levels of lymphocytes can be dangerous and indicate a chronic condition.
Lymphocyte counts can be temporarily high when your body fights an infection. Chronic high or low lymphocyte levels can indicate an underlying problem with your immune system and possibly even cancer.
This article discusses lymphocytes and what they do. It also explains why your levels may be too low (lymphopenia) or too high (lymphocytosis) and what level of lymphocytes is dangerous.
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What Do Lymphocytes Do?
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.
Lymphocytes travel around the body, performing various functions within the lymphatic system. An important part of your immune system, this open network of vessels, organs, and tissues 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 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.
Lymphocytes are also found in lymphoid tissues and organs throughout the body, including your spleen, tonsils, and the lining of your airways.
What Do Lymphocytes Look Like?
Lymphocytes are not visible to the naked eye. When stained and examined under a microscope, they are round and larger than red blood cells. They are composed almost entirely of a dark purple nucleus. The surrounding jelly-like fluid (called the cytoplasm) is a lighter pink.
Types of White Blood Cell Disorders
Types of Lymphocytes
There are two main kinds of lymphocytes: B cells and T cells. They perform different functions in the immune system to protect your body from infection.
Subcategories of B cells and T cells 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 and 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 thethymus. 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.
Hematopoiesis: How All Blood Cells Are Formed
High Lymphocyte Levels
Having too many lymphocytes in your blood is called lymphocytosis. Lymphocytosis is an indication that the immune system has been activated in response to a disease or medical condition.
High lymphocyte levels may be an indication of an infection or cancer of the blood or lymphatic system, among other things.
Lymphocytosis can lead to lymphadenopathy (swollen lymph nodes) and splenomegaly (an enlarged spleen). Splenomegaly can cause dull pain in the upper right portion of the body. Other symptoms may develop depending on the underlying cause.
Causes
Possible causes of lymphocytosis include:
- Acute lymphoblastic leukemia (a type of blood cancer)
- Chronic lymphocytic leukemia (a type of blood cancer)
- Cytomegalovirus (a virus of the herpes family)
- Hepatitis A (a viral infection affecting the liver)
- Hepatitis B (a viral infection affecting the liver)
- Hepatitis C (a viral infection affecting the liver)
- Hypothyroidism (low thyroid function)
- Lymphomas (cancers of the lymphatic system)
- Mononucleosis (caused by the Epstein Barr virus)
- Pertussis ("whooping cough")
- Syphilis (a sexually transmitted bacterial infection)
- Toxoplasmosis (a parasitic infection)
- Tuberculosis (a bacterial infection mainly affecting the lungs)
Treatment
Lymphocytosis is treated by addressing the underlying cause of high lymphocyte levels. Possible treatments for lymphocytosis include:
- Antibiotics
- Antiviral medications
- Chemotherapy
- Immunosuppressants
Some causes of lymphocytosis will resolve on their own with time, and no treatment is needed.
Low Lymphocyte Levels
Having too few lymphocytes is called lymphopenia. Lymphopenia is an indication that lymphocytes are either being depleted or are not being produced in ample quantities.
Low lymphocyte levels may be due to autoimmune diseases, infections, or certain medical treatments. Rare genetic disorders may also interfere with the normal production of lymphocytes.
Causes
Possible causes of lymphopenia include:
- Aplastic anemia (a reduction of red blood cells due to bone marrow failure)
- Chemotherapy
- COVID-19
- HIV
- Hodgkin lymphoma (one of two major types of lymphoma)
- Immunosuppressants (drugs commonly used to treat autoimmune diseases and prevent organ transplant rejection)
- Lupus (a type of autoimmune disease affecting multiple organs)
- Malnutrition
- Radiation therapy
- Severe combined immunodeficiency (a rare inherited disorder characterized by a low immune response)
- Tuberculosis
- Typhoid fever (a tropical bacterial infection)
Treatment
Lymphopenia is treated by addressing the underlying cause.This may include:
- Antibiotics
- Bone marrow or stem cell transplant
- Immunoglobulin
- Immunosuppressant drugs
A low lymphocyte count makes it harder for your body to fight infections. It is important to wash your hands frequently and take other precautions to prevent catching viruses and other communicable illnesses.
Testing
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.
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
Lymphocytes are measured in cells per microliter of blood (cells/mL). Normal lymphocyte counts vary with age and are as follows:
- Adults: 1,000 cells/mL to 4,800 cells/mL.
- Children: 3,000 cells/mL to 9,500 cells/mL
Levels outside the normal range can occur temporarily or indicate a chronic condition. Having too many or too few lymphocytes in your blood can be a sign of infection, autoimmune diseases, and possibly even cancer. Certain medical treatments can also alter your lymphocyte levels temporarily.
Some 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.
Summary
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.
