B cells, or B lymphocytes, are the body’s antibody factories. Apart from that, as memory B cells, they are important for the formation of an immunological memory. They play an essential role in the adaptive immune response.
Definition of B cells
B cells, also known as B lymphocytes, belong to the group of leukocytes (= white blood cells) and as such are an important part of the immune system. Their main task is the production of antibodies.
Function of the B cells in our immune system
Sometimes pathogens manage to overcome the immune system’s external barriers (e.g. the skin) and enter the body. Then the latter immediately takes measures to eliminate it. These measures are summarized as the innate immune response.
However, some pathogens manage to circumvent them. Then another mechanism kicks in: acquired immunity. So-called antibodies are formed as part of acquired immunity, which is also known as adaptive immune response or specific immune response.
Antibodies are produced after the antigen of a pathogen has bound to the appropriate B cell. In this way, they are precisely directed against the pathogen – in contrast to the tools of the innate immune response, which are non-specific and therefore react to all pathogens in the same way.
The only cells in the body that can produce antibodies are the B cells. They are, so to speak, the body’s antibody factories. This makes them one of the most important instruments of the humoral immune response.
The acquired immune system is divided into humoral (= dissolved in the blood serum) and cellular components. At the heart of the humoral immune response is the production of antibodies by B cells. Antibodies are humoral and not cellular because they are proteins.
The cellular immune response, on the other hand, is essentially mediated by T cells. As part of the adaptive immune response, T cells are also pathogen-specific because, like B cells, they have a receptor for recognizing antigens. They are also known as T-lymphocytes, so they belong to the group of lymphocytes like the B-cells.
Structure of B cells
If you look at a blood smear under the light microscope, it is usually not possible to distinguish between naïve B and T cells. Lymphocytes are naïve and have not yet had contact with their antigen. They make up most of the lymphocytes in the blood.
Lymphocytes are characterized by a relatively large cell nucleus in relation to the cytoplasm and are approx. 8 μm in size. However, they can vary in size. A differentiation is only possible when the B lymphocytes have converted into plasma cells. This is because the nucleus of plasma cells is more on the edge of the cell (eccentric), they have more cytoplasm and are somewhat larger overall. However, compared to the naïve lymphocytes, plasma cells are rarely seen in a blood smear.
Figure 1: Comparison of naïve B cells and plasma cells Source:
In order to distinguish the other types of lymphocytes from each other, special methods are required, e.g. B. the so-called flow cytometry. The individual lymphocyte subtypes can be differentiated using laser light.
Formation of B cells
The «B» in B lymphocytes stands for bone marrow, i.e. bone marrow. The bone marrow is the site of formation and maturation of the B lymphocytes.
Originally, the «B» came from bursa fabricii, a lymphatic organ in the cloaca of birds, which serves for the maturation of B-lymphocytes. This cell type was first discovered here. Later, however, B cells were also found in other animals without such an organ. Because the bone marrow is the place where the B lymphocytes form and mature in humans, this designation was retained.
Development of B cells
The formation and maturation of lymphocytes is called lymphopoiesis. Like all blood cells, the B lymphocytes also come from so-called pluripotent stem cells of the bone marrow. Two different types of cells develop from these stem cells as part of blood formation: the lymphoid and the myeloid stem cell.
Among other things, the red blood cells develop from the myeloid stem cell. Lymphocytes develop from the lymphoid stem cell.
The lymphoid stem cell first becomes a pro-B cell. This develops further into the pre-B cell by restructuring the gene segments for the B cell receptor. These restructuring processes are also known as VDJ recombination. This process ensures the diversity of antibodies, which must be sufficient for the large number of different antigens.
The B-cell receptor corresponds to a membrane-bound antibody (IgM or IgD) which, like a soluble antibody, can also bind antigens. Binding to the membrane occurs via a protein (CD79) that crosses the cell membrane and plays an important role in the transmission of signals.
Each B cell carries many B cell receptors. However, all receptors of a B cell are directed against the same antigen.
After the completion of the B-cell receptor, the so-called negative selection takes place. Here, cells with autoreactive B-cell receptors that bind to endogenous antigens are eliminated. Surviving cells are self-tolerant. This means that they do not perceive the body’s own antigens as foreign and harmful.
The B cell now leaves the bone marrow. However, since their receptor has not yet come into contact with an antigen, this is a naïve B cell. This naïve B cell then searches for the right antigen in the blood and in the secondary lymphatic organs (e.g. the lymph nodes).
If it finds its antigen there, it is activated.
Figure 2: Simplified representation of B cell development Source:
Activation of B cells
In order for a B lymphocyte to be able to produce antibodies, it must be activated. There are basically two different ways of activating the B cells:
- the T-cell dependent activation,
- T-cell independent activation.
T cell independent activation
B cells are activated by encountering their antigen and binding to it with their receptor. As a result, the B cell multiplies (= proliferates) and develops into a plasma cell (= differentiation), which then forms the specific antibodies.
Memory cells, i.e. cells that remember their antigen for years and can thus quickly induce an immune response, are not formed here.
T cell dependent activation
For many antigens, another requirement for activation must be met: co-stimulation by T cells.
T-cell dependent activation takes place in the germinal centers of secondary lymphoid organs. The B lymphocyte takes up the antigen and presents it to a T helper cell on its surface with the help of a special molecule, the MHC class II complex.
If the T helper cell matches the antigen, it can bind to this antigen-MHC complex. As a result, the T helper cell releases certain substances, the so-called cytokines. The B cell is activated by the cytokines and clonal expansion (multiplication) occurs. The cloned B cells then develop into plasma cells as in T cell independent activation. In addition, some of the B cells also develop into the B memory cells already mentioned.
The type of B cell activation required depends on the type of antigen.
B cell functions
B cells have different roles in the immune system, all of which ultimately aim to produce antibodies. Essentially, this involves:
- antigen presentation,
- antibody production and
- formation of memory cells.
The functions of the B cells are explained in more detail below.
Antigen presentation by B cells
One task of the B lymphocytes is antigen presentation. Here, the B cell takes up the antigen after binding to the B cell receptor via endocytosis and presents it on its surface. To do this, the antigen binds to an MHC II complex. The antigen presentation is therefore the prerequisite for the activation of the lymphocytes described above. The activation in turn leads to the formation of antibodies.
antibody production
As you have already learned, once activated, B lymphocytes differentiate into antibody-producing plasma cells. The antibodies are directed against the antigens of the foreign structures and represent an important weapon of the specific immune system. They fulfill their defense function in different ways.
For example, they neutralize the antigen, rendering the pathogen harmless. Antibodies also help phagocytes by labeling the antigen-bearing cells or by clumping them together so the phagocytes can eliminate them more easily. Furthermore, antibodies activate the complement system and in this way lead to the destruction of the pathogen.
In passive immunization, a person who does not have antibodies of their own is given antibodies from an immune person for short-term protection against infection.
Formation of memory B cells
The development of memory B cells is another reason for the effectiveness of the specific immune system. Because when these long-lived cells recognize their antigen again after some time, they quickly induce the formation of antibodies. For example, a virus has B. less time to multiply and cause a serious infection.
Figure 3: B cell functions Source:
B cell defects
As part of the immune system, B cells serve to maintain health. Sometimes they can also be the cause of disorders in the body.
autoimmune diseases
As a rule, during maturation, B lymphocytes are destroyed, the antibodies of which would be directed against the body’s own healthy cells. If there is an error in control and B lymphocytes without self-tolerance survive the selection process, then an autoimmune disease can develop. The antibodies attack harmless, endogenous cells.
An example of a disease in which autoreactive antibodies are involved is disease Hashimoto’s thyroiditis. The antibodies attack the cells of the thyroid gland. As a result, the thyroid can no longer produce enough hormones and, over time, symptoms such as tiredness, intolerance to cold and a slow heartbeat (bradycardia) occur.
One option for treating autoimmune diseases is cell depletion, which involves removing B cells from the body, for example. This prevents the formation of autoreactive antibodies. However, the absence of antibody-producing cells weakens the immune system, making it harder for the organism to defend itself against pathogens.
B-cell lymphomas
Other diseases that can originate from B cells are B cell lymphomas. This is a form of cancer in which B cells multiply uncontrollably…