B Cells (B Lymphocytes) - The Definitive Guide | biology dictionary (2023)

Definition

B cells or B lymphocytes are part of the adaptive immune response. Once activated, these white blood cells produce antibodies. B lymphocytes play additional roles as antigen-presenting and cytokine-secreting cells. This cell type is classified into four major groups: transitional, naïve, plasma, and memory B cells.

What are B cells?

B cells are lymphocytes, a type of white blood cell. They are the result of multipotent cell differentiation in the bone marrow.

Every blood cell is derived from a single cell type: the pluripotent hematopoietic stem cell. This stem cell changes or differentiates in two ways in the bone marrow. One is the common myeloid progenitor that produces white blood cells, platelets, and red blood cells. The other is the common lymphoid progenitor. B cells and T cells develop from the common lymphoid progenitor cell.

What do B cells do?

The function of B cells is threefold:

• Antigen presentation to other immune cells
• cytokine secretion
• antibody production

Before a B lymphocyte is activated, it does very little. In fact, up until the 1960's, most textbooks told us that there was no function of B cells. Read quickly for an interesting history of lymphocyte researchthis scientific article.

antigen presenting cells

Although the most important function of B cells is our humoral (antibody-driven) immunity, inactivated B cells also function as antigen presenting cells (APCs).

Antigen presenting cells are found throughout the body. They bind to an antigen (foreign particle) and process it, producing membrane markers that act as a warning (the antigen-MHC complex in the diagram above) that a T cell can understand.

B lymphocytes present these antigen-MHC complexes to T cell receptors, causing T cell activation.

For the non-specific immune system (the innate immune system), this response produces the opposite effect: inactivating T cells, suggesting that B lymphocytes also help prevent autoimmune reactions.

cytokine secretion

Cyto (Greek for cell) and kinos (Greek for movement) describe the effect of cytokines: they cause cell movement. Cytokines are signaling molecules and are essential for cell-to-cell communication.

When B lymphocytes release cytokines, they invite white blood cells in the form of phagocytes to areas where B cell antibodies have bound to antigens.

production of antibodies

The main function of B cells is the production of antibodies. To understand this phenomenon, it is important to know something about the process of humoral immunity.

Humoral immunity begins in the B lymphocytes. While still in the bone marrow, a B cell develops specialized membrane receptors called B cell receptors (BCRs). These proteins are the equivalent of locks matching antigenic keys. The B lymphocytes then migrate to the lymphatic organs. Once there, they are called naïve B cells.

When a virgin B cell comes in contact with an antigen that matches its receptors, it binds to it and transports it to its membrane for processing (endocytosis). This processing is important because it leads to the formation of antigen-MHC complexes that T cells can recognize.

contact with an antigenit is notCausing B Cell Activation When a T cell binds to the antigen-MHC complex, it releases cytokines. These T cell cytokines activate B lymphocytes.

Activation means that the B lymphocyte divides to form one of two types of daughter cells; activated plasma cells or inactive storage cells.

Memory cells live longer and recognize the same antigen when it later attacks. Plasma cells respond immediately by releasing antibodies. Tiny antibodies circulate in the blood ready to bind to your specific type of antigen.

How do antibodies work?

Antibodies or immunoglobulins are divided into five classes:

• IgM: first reaction; causes many types of antigens to clump together
• IgA: protects against mucosal pathogens
• IgD: Receptor function: Signals activation of B cells, which are normally co-expressed with IgM
• IgG - the most common human immunoglobulin - marks a variety of pathogens for elimination
• IgE: binds to mast cells and basophils and causes the release of histamine, which is linked to allergies.

An antibody can inactivate an antigen by three different processes.

The first is complement fixation. During complement fixation, the antibody binds to a foreign particle and breaks it down. This process also attracts other white blood cells through chemotaxis (chemical messages - cytokines). Antigens are destroyed by opsonization (antibodies mark them so other cells can recognize and destroy them).

Another way antibodies attack antigens is through neutralization. When an antibody binds to an antigen, it stops the antigen from releasing toxins.

The last method is agglutination, in which the antibodies ensure that the foreign particles stick together. Once grouped in a group, it is easier for phagocytes (digestive cells) to eliminate them. This is how IgM antibodies work.

B cell types

There are four main types of B cells: transitional, naïve, plasma, and memory cells.

transitional B cells

A transitional B cell is the link between immature B lymphocytes in the bone marrow and mature B cells in the lymphoid organs. These cells differentiated from common myeloid progenitor cells in the bone marrow into B lymphocytes; however, they are not yet mature.

The maturation process takes place in the lymphatic organs. Between bone marrow production and maturation (NOactivation) in the lymphatic organs, a B cell is called a transitional B cell.

Very few immature B cells live long enough to mature. It is believed (but not yet proven) that too many transitional B cells can trigger autoimmune diseases such as lupus erythematosus and rheumatoid arthritis.

naïve B cells

Naïve B cells are found in secondary lymphoid organs. They are mature but not activated yet. Naïve B lymphocytes can differentiate into plasma or memory B cells.

Another function pairs naïve B cells with naïve T cells to prevent the latter from multiplying. This created a new subcategory of B cells: Breg cells (regulatory B cells). We're just beginning to learn about them. As the full name suggests, Bregslimit the humoral immune responsedecreasing T cell responses.

plasma cells

Plasma cells or effector B cells are the cloned daughter cells of activated naïve B cells. Plasma cells produce antibodies, but this is a fairly slow response to antigen detection.

Two processes are required to produce plasma cells. First, a naïve B cell must present an antigen to a helper T cell. The now activated T cell must in turn bind the B cell. This double authentication turns a naïve B cell into an activated B cell. Only an activated virgin B cell can divide to produce plasma cells or memory cells.

We don't always need helper T cells to elicit a generalized humoral response. B cells can be activated and divide to form daughter cell clones that can only secrete IgM antibodies through a process called T-cell-independent antigenic stimulation.

Plasma cells secrete antibodies that bind to the type of antigen first processed by the naïve B cell. T-cell independent activation only produces IgM immunoglobulins. A plasma cell cannot secrete more than one type of antibody. Plasma cells move around the body according to the distribution of cytokines and secrete antibodies closer to the source of attack.

storage cells

A small percentage of plasma cells differentiate into memory cells. Memory B cells have a much longer lifespan (years) than plasma cells (days to months). We still don't know what causes a B cell to differentiate.

Memory cells spread throughout the body. Like plasma cells, they have a specific affinity for a particular type of antigen. Their longer lifespan means that when plasma cells have completed their task and an antigen has been defeated, a second attack at a later date elicits a much faster response.

A second exposure to the same antigen causes memory B cells to divide to form plasma cells. This happens quickly and is called a secondary immune response.

B1 and B2 cells

The B lymphocytes discussed in this article are B2 cells derived from pluripotent hematopoietic stem cells from the bone marrow.

Recent research has found another much smaller group of immune cells: the B1 cells. These develop in the yolk sac of an embryo. Like B2 cells, they produce antibodies.

However, B1 cells live much longer than B2 cells and are able to regenerate. They are also polyspecific, meaning they can bind to different antigens. Adults do not appear to produce B1 cells, but B1's regenerative abilities mean these cells are found in adults; B1 populations generally remain stable from birth.

B cells vs. T cells

T cells and B cells contribute to our specific immunity and develop from the same progenitor stem cell. However, they have many differences.

• B lymphocytesdifferentiateWithinBa bone marrow; T lymphocytes differentiate intoThimus, which are the primary lymphatic organs.
• T lymphocytesmaduroin secondary lymphoid organs such as lymph nodes, spleen, tonsils and cecum; B lymphocytes mature in the bone marrow.
• There are many more T cells than B cells. T cells make up about 80% of all circulating lymphocytes.
• T lymphocytes tend to be more diffusely distributed in lymph nodes; B lymphocytes congregate in more compact groups.
• B cells are the basis of humoral immunity. Humoral immunity is antibody-mediated immunity; T cells are part of our cell-mediated immunity and activate other immune cells.
• T cells live much longer
• T cells cannot recognize an antigen unless it is presented by an APC. The B cell does not always need a T cell to become activated.
• When activated, B lymphocytes become antibody-secreting plasma cells or memory cells; T lymphocytes become one of three types of effector T cells:

• • Cytotoxic T Cells: Kill infected cells
  • Helper T cells: activate naïve B lymphocytes
  • Regulatory T cells: stop an immune response when the threat has passed

B cell lymphoma

Blood cancer affects the production of blood cells.Lymphoma or cancer of the cells of the lymphatic systemIt most commonly affects the development and production of B lymphocytes.

B-cell non-Hodgkin's lymphoma (NHL) accounts for 80% of all non-Hodgkin's cases. This type of cancer is divided into:

• Diffuse large B-cell lymphoma (DLBCL): Usually affects older populations. rapid growth High healing rate.
• Follicular Lymphoma: Usually affects older populations. slow growth
• Mantle cell lymphoma (MCL): usually affects older men. Fast growing and difficult to treat.
• Marginal Zone Lymphoma: Usually affects older populations. slow growth
• Burkitt lymphoma: usually affects male children. rapid growth 50% healing rate.

Treatment for B-cell lymphoma is usually limited to chemotherapy; It is impossible to irradiate the whole body.

DLBCL responds well to a combination of cytostatics, steroids, and monoclonal antibodies called R-CHOP, a cocktail of rituximab, cyclophosphamide, doxorubicin, vincristine, and prednisone.

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