What are lymphocytes?
The lymphocytes belong to the white blood cells, the leukocytes, and make up about 20 to 45 percent of the leukocytes in the blood.
However, the majority of lymphocytes reside in the so-called secondary lymphatic organs . These include:
- spleen
- lymph nodes
- Mucous membrane-associated lymphoid tissue (MALT) – this is a tissue that lies under the mucous membrane of various organs and serves the immune defense there. This tissue is found in various parts of the body and contains lymphocytes.
Lymphocytes in the superficial layer (in the epithelium) of the intestinal mucosa are also called intraepithelial lymphocytes. Their function is not exactly clear. They are elevated in the context of celiac disease (also called sprue).
B lymphocytes can be distinguished from T lymphocytes . These names go back to the place where the cells are formed: B lymphocytes arise in the bone marrow and T lymphocytes in the thymus. These two regions of the body are referred to as the primary lymphoid organs .
The task of the B and T lymphocytes is to recognize and fight pathogens. They are part of the acquired immune system, which only develops over the years and through contact with the smallest particles in our environment. Unlike the cells of the innate immune system (e.g. granulocytes and monocytes), lymphocytes specialize in certain antigens that they can recognize and fight. Antigens are the smallest structures – mostly proteins – on and in foreign cells and pathogens.
The so-called natural killer cells (NK cells) also belong to the lymphocytes. However, they are part of the innate immune system.
How does the immune system work through lymphocytes?
In the so-called primary lymphatic organs, bone marrow and thymus, T and B lymphocytes are formed and mature. Matured lymphocytes circulate in the blood and reside in the secondary lymphatic organs (spleen, lymph nodes and lymphatic tissue of the mucous membranes; MALT). They “patrol” so to speak in different areas of the body and are there for the immune system.
Lymphocytes have receptors on their surface with which they can recognize antigens. Each lymphocyte is specialized for an antigen. When it encounters and binds to the appropriate antigen, the lymphocyte is activated and begins to multiply.
Once activated, T lymphocytes can have two functions: either they help other cells to fight pathogens (they are also called T helper cells), or they cause the cell death of infected cells.
On the other hand, the B lymphocytes are responsible for the production of antibodies: some of their receptors (the so-called B cell receptors) are antibodies ( immunoglobulins ) that can bind to antigens. After their activation, the antibodies of the B lymphocytes are even better matched to the antigen. In the next step, the B lymphocytes produce these antibodies and release them into their environment. They are now called plasma cells. The antibodies bind the antigen and ensure that it is broken down and cannot harm the body.
Both types of lymphocytes develop so-called memory cells after an infection . In this way, they can react more quickly to a new contact with the same pathogen and start fighting it. The same effect is also achieved, for example, by vaccination.
When and how is the lymphocyte value determined?
The lymphocytes are determined as part of a complete blood count . This examination is carried out, for example, if there is a suspicion of an infection, an autoimmune disease or problems with blood formation. Inflammation also has the consequence that the number of leukocytes (and thus also lymphocytes) suddenly increases, which is why the blood value is often determined when inflammation is suspected.
This analysis is also referred to as a differential blood count. Usually a drop of blood is examined under the microscope. Not only can one determine the number of lymphocytes, their ratio to the leukocytes as a whole and their condition and shape. Other blood values, such as erythrocytes or thrombocytes, can also be determined in this way.
What is a normal lymphocyte count?
The lymphocytes (also: “lymphos”) are determined in the blood count. Either the absolute number of lymphocytes per microliter of blood (alternatively: per liter or nanoliter) or their proportion of leukocytes (relative) is given. The standard values vary depending on age and can be set at different levels from laboratory to laboratory. Some standard values are summarized in the following table:
Lymphocytosis – when there are increased lymphocytes in the blood
If the lymphocyte value in the blood is too high, this is referred to as lymphocytosis. This can be relative if the proportion of lymphocytes in adulthood is over 50 percent, or absolute if the number of lymphocytes in adulthood is over 4,000 per microliter (corresponds to 4 per nanoliter).
The increased value indicates that the immune system is fighting an infection. However, certain types of blood cancer can also cause increased lymphocytes. If the lymphocytes also change their appearance, this is referred to as atypical lymphocytosis.
Causes for an increased number of lymphocytes can be, for example:
Elevated lymphocyte levels can usually be reduced by treating the underlying disease.
When is the lymphocyte count too low?
If the number of lymphocytes is too low, one speaks of lymphocytopenia or lymphopenia. This can also be relative (lymphocyte percentage below 20 percent) or absolute (lymphocyte number below 1,000 per microliter or 1 per nanoliter).
Lymphocytopenia is a sign that the immune system is weakened. The body is thus more susceptible to disease, which makes low lymphocyte levels worrying. There are several reasons for low lymphocytes:
Drugs that suppress the immune system (immunosuppressors)
- Cytotoxic chemotherapies
- radiotherapy
- Congenital defects of the immune system (e.g. Fanconi anemia, Di George syndrome)
- HIV/AIDS
- Some autoimmune diseases (such as lupus erythematosus)
- Cancer diseases (including lymphomas , acute lymphatic leukemia )
Low lymphocyte counts can also occur as part of an infection with the SARS-CoV-2 coronavirus . Worldwide, the most common cause of insufficient lymphocytes is protein malnutrition.
In order to increase the lymphocyte count, the underlying disease must be treated. If a drug is to blame for the low lymphocytes, the doctor treating you must weigh up the benefits and risks of the therapy and adjust it if necessary.
Autoimmune diseases and lymphocytes
As already mentioned, certain autoimmune diseases can alter the lymphocyte count. However, lymphocytes also play a role in the development of some autoimmune diseases. Hashimoto ‘s thyroiditis , a disease of the thyroid gland, is caused by T-lymphocytes, and rheumatoid arthritis also develops autoimmune traits as the disease progresses, which are caused by T-lymphocytes. Researchers have also found that a certain type of innate lymphocyte (ILC2) can be protective against rheumatoid arthritis. 1
