Lymphocytes are white blood cells that are also one of the body's main types of immune cells. They are made in the bone marrow and found in the blood and lymph tissue.
The immune system is a complex network of cells known as immune cells that include lymphocytes. These cells work together to defend the body against foreign substances, such as bacteria, viruses, and cancer cells that can threaten its functioning.
In this article, we look at different types of lymphocytes, what normal levels to have in the blood are, and what happens if levels get too low or too high.
There are two categories of lymphocytes known as B lymphocytes and T lymphocytes. These are commonly referred to as B cells and T cells.
The job of B cells is to make antibodies, which are proteins produced by the immune system to fight foreign substances known as antigens.
Each B cell is set to make one specific antibody. Each antibody matches an antigen in the same way that a key matches a lock, and when this happens, the antigen is marked for destruction.
The job of T cells is to help the body kill cancer cells and control the immune response to foreign substances. They do this by destroying cells in the body that have been taken over by viruses or become cancerous.
A third type of lymphocyte, known as a natural killer or NK cell, comes from the same place as B and T cells. NK cells respond quickly to several foreign substances and are specialized in killing cancer cells and virus-infected cells.
There are different types of B cells and T cells that have specific roles in the body and the immune system.
Memory B cells
Memory B cells circulate in the body to start a fast antibody response when they find a foreign substance. They remain in the body for decades and become memory cells, which remember previously found antigens and help the immune system respond faster to future attacks.
Regulatory B cells
Regulatory B cells or Bregs make up around 0.5 percent of all B cells in healthy people. Although few in number, they have a vital role to play.
Bregs have protective anti-inflammatory effects in the body and stop lymphocytes that cause inflammation. They also interact with several other immune cells and promote the production of regulatory T cells or Tregs.
Killer T cells
Killer or cytotoxic T cells scan the surface of cells in the body to see if they have become infected with germs, or if they have turned cancerous. If so, they kill these cells.
Helper T cells
Helper T cells "help" other cells in the immune system to start and control the immune response against foreign substances.
There are different types of helper T cells, and some are more effective than others against different types of germs.
For instance, a Th1 cell is more effective against germs that cause infection inside other cells, such as bacteria and viruses, while a Th2 cell is more effective against germs that cause infection outside of cells, such as certain bacteria and parasites.
Regulatory T cells or Tregs
Tregs control or suppress other cells in the immune system. They have both helpful and harmful effects.
They maintain tolerance to germs, prevent autoimmune diseases, and limit inflammatory diseases. But they can also suppress the immune system from doing its job against certain antigens and tumors.
Memory T cells
Memory T cells protect the body against previously found antigens. They live for a long time after an infection is over, helping the immune system to remember previous infections.
If the same germ enters the body a second time, memory T cells remember it and quickly multiply, helping the body to fight it more quickly.
Natural killer T cells
Natural killer T cells are a mixed group of T cells that share characteristics of both T cells and natural killer cells. They can influence other immune cells and control immune responses against substances in the body that trigger an immune response.
Lymphocyte levels can change according to a person's race, gender, location, and lifestyle habits.
Unusually high or low lymphocyte counts can be a sign of disease.
Lymphocyte counts above the normal range can be a harmless and temporary situation due to the body's normal response to an infection or inflammatory condition.
But a high level of lymphocytes can also be a sign of lymphocytosis, which is a more serious condition.
Lymphocytosis is frequently associated with chronic infections, some blood cancers, and with autoimmune diseases, such as inflammatory bowel disease.
In adults, lymphocytosis usually corresponds to a lymphocyte count higher than 3,000 lymphocytes in 1 µL of blood. In children, the lymphocyte count would be around 9,000 lymphocytes in 1 µL of blood, although this value can change with age.
A low level can also be a sign of a condition known as lymphocytopenia or lymphopenia.
Lymphocytopenia can be inherited, or it can be acquired alongside certain diseases, including:
- rare inherited diseases, such as ataxia-telangiectasia
- nerve diseases, such as multiple sclerosis
- autoimmune diseases
- AIDS, or other infectious diseases
Lymphocytopenia can also be a side effect of medications or some other medical treatments.
A blood test that counts how many lymphocytes are in a person's blood is called a B and T cell screen. In this test, the levels of the main types of white blood cells in the body are measured.
Lymphocyte count is one part of a larger whole blood test called a complete blood count (CBC). A CBC can be requested by doctors if they suspect that a disease or infection is present.
A sample of the bone marrow can also be used instead of blood in some cases.
The B and T cell screen will give an estimate of the amount of T and B cells in the blood.
Results can indicate a normal cell count or an abnormal cell count, the latter pointing to the possible presence of a disease. In this case, the doctor will likely ask for other tests to confirm a diagnosis.
T cell counts above the normal range can indicate any of the following conditions:
- sexually transmitted disease, such as syphilis
- viral infection, such as infectious mononucleosis
- infection caused by a parasite, such as toxoplasmosis
- tuberculosis, a disease that affects the lungs and other organs
- cancer of the white blood cells
- cancer of the blood, starting in the bone marrow
B cell counts above the normal range can indicate:
- chronic lymphocytic leukemia
- multiple myeloma
- a genetic disease known as DiGeorge syndrome
- a type of cancer called Waldenstrom macroglobulinemia
T cell counts below the normal range can indicate:
- a disease present from birth
- an acquired T cell deficiency disease, such as HIV, which can progress to AIDS or HTLV-1
- a type of cancer
B cell counts below the normal range can indicate:
- acute lymphoblastic leukemia
- HIV or another disease that weakens the immune system
Unusually high or low lymphocyte counts may cause no signs, symptoms, or serious problems on their own. They can be the body's normal response to an infection, inflammatory condition, or other unusual condition, and will return to normal levels after some time.
If lymphocyte counts remain high or low over time, they could be a sign of a health condition and may be diagnosed as lymphocytopenia or lymphocytosis. These conditions can range from mild to severe, and their duration depends on what caused them.
Treatment for abnormal levels of lymphocytes will depend on both the cause and severity and mild forms may not require any at all.