B lymphocytes (B cells) are among the white blood cells (leukocytes) and are the only cells that can also produce antibodies. If activation by foreign antigens occurs, they differentiate into memory cells or plasma cells.
What are B lymphocytes?
B lymphocytes are classified as part of the white blood cell group. Their most important function is the formation of antibodies. Discovered for the first time in birds, B lymphocytes in humans are formed in the bone marrow or fetal liver. B lymphocytes make up about five to ten percent of the lymphocytes circulating in the blood. They are predominantly found in the bone marrow, lymph nodes, spleen, and lymphoid follicles.
Function, action, and tasks
The human immune system can be divided into three parts:
- Surface barriers such as skin or mucous membranes.
- Internal defenses against inflammation and fever
- Adaptive defenses
In this context, adaptive defenses include T lymphocytes and B lymphocytes, and these defense mechanisms can be divided into cell-mediated and humoral immunity, respectively. B lymphocytes play an essential role in immune defense. The term B cell comes from the English term “bone marrow“. If there is contact with a foreign pathogen, so-called immune globulins are formed in the B lymphocytes. An antibody is formed against each antigen, whereby the B lymphocytes concentrate mainly on toxins and bacteria. Antibodies are special proteins that can be found in various body fluids. Antibodies protect the body from:
- Viruses
- Bacteria, fungi
- Foreign and tumor tissue
- Animal toxins
- Flower pollen
- Artificial and natural substances
If a division of B lymphocytes occurs, plasma cells are formed. Some of them exist only for a few weeks, others are among the memory cells and remain in the human body for years. These are also called memory B cells. In addition, based on their function, B lymphocytes are also divided into plasmablasts and naive B cells, respectively. Plasmablasts are activated B-lymphocytes, whereas non-activated B-cells are found in the lymphatic system or in the bloodstream. If these perceive an antigen, it is taken up and subsequently released as a protein complex.
Formation, occurrence, properties, and optimal levels
Initially, a mature B lymphocyte circulates in the bloodstream as well as in the lymphatic system. When it comes into contact with an antigen, the antigen is bound to the B-cell receptor. This process is called receptor-mediated endocytosis. The antigens can thus enter the acidic cell compartments, where they are cleaved to peptides. This is followed by transport to the cell surface. However, binding alone is not sufficient for activation of the B lymphocyte. Only if the antigen is also recognized as foreign by a T helper cell can the B lymphocyte be activated and antibodies produced. Basically, B cells require two signals for activation. The first they receive via the binding of the receptor, the second via the binding of CD4oL to CD40. After activation, the B lymphocyte travels to the nearest lymph node, where it differentiates into plasma cells. These then form antibodies. Plasma cells have an oval to spherical shape, their nucleus is usually eccentric and they are strongly basophilic. Mature plasma cells are found in the spleen, bone marrow, lymph node marrow, exocrine glands, mucous membranes, and chronic inflammatory sites. A smaller proportion develops into memory B cells, which circulate in the lymphatic system or in the blood even after a defended infection. If an antigen now enters the body again, the immune reaction is faster because the blueprint for the corresponding antibodies is already known. The information about the structure of the antibodies can be found in the DNA of the B lymphocytes. Since the human body comes into contact with billions of different antigens, there are also a wide variety of lymphocyte clones that have different DNA codes. In addition to the different terminal and mature stages of B lymphocytes, there are basically two types of B cells: B2 cells are referred to as “common” B cells, while B1 cells are larger and mainly found in the abdominal cavity.These cells are not present in the peripheral lymph nodes. They are also distinguished from B2 cells by certain surface markers.
Diseases and disorders
An increase in B lymphocytes may be seen in the following diseases:
- Certain infectious diseases
- Autoimmune diseases
- B-cell lymphomas (for example, chronic lymphocytic leukemia).
Decreased values, on the other hand, occur in the following diseases:
- Liver disease
- Iron deficiency
- Immunodeficiencies
In the context of B-cell lymphoma, a proliferation of a group of lymphocytes occurs at one site in the body, which is also known as clonal growth. In this case, it is possible that the disease is confined to lymphoid tissue, but the lymphocytes can also spill over into the blood, in which case it is referred to as lymphocytic leukemia. There are two groups of lymphomas:
- Non-Hodgkin’s lymphoma (NHL).
- Hodgkin’s lymphoma
Non-Hodgkin’s lymphomas can in turn be divided into B-cell NHL as well as T-cell NHL. B-cell lymphomas include, for example:
- Immunocytomas
- Multiple myelomas
- Chronic lymphocytic leukemia
Chronic lymphocytic leukemia is very common in this case, with the following symptoms:
- General weakness
- Rashes, itching
- Swelling of the lymph nodes
- Enlargements of the liver and spleen
