The white blood cells that are involved in an acquired immune response are called lymphocytes. There are two types of lymphocytes - B-cells and T-cells. Both of these cells are continually produced in the bone marrow.
These cells are not involved in the immune response until they are fully developed. The B-cells mature in the bone marrow, while T-cells migrate through the bloodstream and mature in the thymus gland.
B-cells help fight against bacteria and viruses that enter the body. They are able to detect and bind to specific foreign invaders (antigens) that enter the body. This triggers other immune cells to destroy the antigen. Naïve B-cells are B-cells that have not encountered an antigen yet. Once a B-cell has been exposed to an antigen, it is called a plasma cell.
The plasma cell secretes proteins called antibodies on its surface. There are five classes of antibodies (also called immunoglobulins): immunoglobulin A (IgA), immunoglobulin D (IgD), immunoglobulin E (IgE), immunoglobulin G (IgG), and immunoglobulin M (IgM). Each antibody is specific for a certain antigen.
The antibody binds to the surface of the antigen, triggering other immune system cells to destroy it. Once linked to the antigen, the B-cell sends a signal that attracts helper T-cells. The T-cell binds to the receptor on the B-cell, which stimulates the B-cell to produce many plasma cells. Each plasma cell produces millions of identical antibodies, which are released into the bloodstream.
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types of b‑cells
Naïve B-cells: Naïve B-cells are present in the bloodstream. They are mature B-cells that have not been exposed to an antigen yet. These cells have antigen-specific antibodies immunoglobulin M (IgM) and immunoglobulin D (IgD) on their surfaces. These naïve B-cells are able to recognize their cognate antigen. Since there are millions of B-cells in the body, and naïve B-cells only live a few days, more than 90% of these cells die before they come into contact with an antigen.
Plasma B-cells: Plasma B-cells, or plasma cells, are large B-cells that have been exposed to an antigen. They secrete large amounts of antibodies. They are sometimes called antibody factories. Plasma B-cells have large amounts of rough endoplasmic reticulum, which is a system of membranous tubes and sacs containing ribosomes that produce the membrane-bound antibodies. These plasma cells are found in the spleen and lymph nodes.
Memory B-cells: Memory B-cells are formed from activated B-cells. These cells are specific to an antigen that has previously entered the body. Memory B-cells, which are present in the bone marrow, lymph nodes, and spleen, are able to respond quickly when they are exposed to the same antigen in the future. Lower levels of the antigen are able to activate memory B-cells better than naïve B-cells. Therefore, the memory B-cells enable the immune system to react more quickly if their cognate antigen enters the body in the future. Memory B-cells have a prolonged life span and they can survive for many years (up to a lifetime).
B-1 cells: B-1 cells express the CD5 protein on their surfaces, which can bind to another protein called CD72. It has been suggested that the CD5-CD72 link mediates interaction among B-cells.
B-1 cells express more immunoglobulin M (IgM) than immunoglobulin G (IgG) and their receptors show polyspecificity. This means that they are able to identify several different antigens if they are present in high quantities. However, they have a preference for other immunoglobulins, self-antigens, and common bacteria. These cells respond to antigens that are T-cell independent. This means that the B-1 cells are activated without the help of T-cells.
Most B-1 cells are found in the abdominal and chest cavities. Low quantities of B1-cells are also found in the lymph nodes and spleen.
B-2 cells: B-2 cells are considered conventional B-cells. They respond to antigens that are T-cell dependent. This means that T-cells are needed to activate the B-1 cells.
development and function
Millions of B-cells are produced daily in the bone marrow.
Once the B-cell reaches maturity, it has a B-cell receptor (BCR) on its surface. The BCR is a protein that distinguishes the B-cell from other lymphocytes (immune system cells). At this stage, the B-cells can express (but do not secrete) immunoglobulin M (IgM) and immunoglobulin D (IgD) on their cell surfaces. This mature cell is now able to respond to antigens.
The mature B-cells circulate in the bloodstream and lymph nodes, searching for foreign substances like bacteria and viruses that may harm the body. These B-cells are also called naïve cells because they have not encountered an antigen yet. These cells can be activated in either a T-cell dependent or independent manner.
After a B-cell detects and binds to an antigen, it internalizes it. The B-cell then attaches parts of the antigen's proteins to a major histocompatibility complex (MHC) class II molecule. This complex is then transported to the outside of the cell membrane where a T-cell can identify it. Once the T-cell identifies the processed antigen, it binds to the complex on the B-cells surface. This process activates the T-cell to produce cytokines, which are chemical messengers for cells. When cytokines are produced, the B-cell is activated to produce more antibodies against the antigen.
The activated B-cell begins to divide, and it becomes an antibody-producing cell called a plasma cell.
The plasma cell produces immunoglobulin that is secreted on the cell surface. Some of the daughter cells of the plasma cell undergo isotype switching. During this process, other isotypes of immunoglobulin, including IgA, IgE ,and IgG, are secreted.
If the B-cell starts to mature abnormally, it will die in a process called apoptosis (programmed cell death).
The main difference between B-cells and T-cells is how they recognize antigens. B-cells are able to recognize their cognate antigen in its unprocessed form via immunoglobulins on their surfaces.
T-cells, on the other hand, identify their cognate antigen in a processed form. An antigen-presenting cell, like a B-cell, macrophage, or dendritic cell, must first break down the antigen. The T-cell receptor recognizes the processed antigen.
t‑cell dependent activation
Most antigens are T-cell dependent. This means that T-cells are needed in order for the most antibodies to be produced. This type of immune response is also called cell-mediated immunity.
If an antigen is T-cell dependent, the first signal occurs when the B-cell receptor detects and binds to the antigen. This triggers the B-cell to internalize and digest the antigen. It attaches the antigen to a MHC (major histocompatibility complex) class II protein and presents it to a special type of T-cell called a TH2-cell. The T-cell detects and binds to the antigen-MHC complex on the surface of the B-cell. This triggers the T-cell to release chemical messengers called cytokines.
The second signal occurs when cytokines are released. This triggers the B-cells to multiply and become plasma cells. The plasma cells are able to produce many different types of immunoglobulin, in a process called isotype switching. Memory B-cells are also produced in response to T-cell dependent antigens.
t‑cell independent activation
Antigens may also be T-cell independent. This means that T-cells are not needed to activate the B-cells. There are two types of T-cell independent activation -
type 1 T cell-independent activation and type 2 T-cell independent activation.
During type 1 activation, the antigen is a polyclonal activator. This means the antigen is non-specific to a particular antibody. Therefore, the antigen activates a large percentage of the B-cells at one time because many different B-cells are able to recognize the antigen.
During type 2 activation, a minimum number of identical antigens are needed to trigger the B-cell. During this process, macrophages (another type of white blood cell) present many antigens to the B-cell, which triggers the antibodies on the surfaces of B-cells to cross-link.