B cells or B lymphocytes

B lymphocytes, also known as B cells, are one of the five types of white blood cells, or leukocytes, that circulate throughout the blood. They and T-lymphocytes are the most abundant types of white blood cells. B lymphocytes are a vital part of the body's immune system . They function to specifically recognize a foreign protein, designated as an antigen , and to aid in destroying the invader.

B lymphocytes are produced and mature in the bone marrow. The mature form of the cell is extremely diverse, with a particular B cell being tailored to recognize just a single antigen. This recognition is via a molecule on the surface of the B cell, called a B cell receptor. There are thousands of copies of the identical receptor scattered over the entire surface of a B cell. Moreover, there are many thousands of B cells, each differing in the structure of this receptor. This diversity is possible because of rearrangement of genetic material to generate genes that encode the receptors. The myriad of receptors are generated even before the body has been exposed to the protein antigen that an individual receptor will recognize. B cells thus are one means by which our immune system has "primed" itself for a rapid response to invasion.

The surface receptor is the first step in a series of reactions in the body's response to a foreign antigen. The receptor provides a "lock and key" fit for the target antigen. The antigen is soluble; that is, floating free in the fluid around the B cell. A toxin that has been released from a bacterium is an example of a soluble antigen. The binding of the antigen to the B cell receptor triggers the intake of the bound antigen into the B cell, a process called receptor-mediated endocytosis. Inside the cell the antigen is broken up and the fragments are displayed one the surface of the B cell. These are in turn recognized by a receptor on the surface of a T lymphocyte, which binds to the particular antigen fragment. There follows a series of reactions that causes the B cell to differentiate into a plasma cell, which produces and secretes large amounts of an antibody to the original protein antigen.

Plasma cells live in the bone marrow. They have a limited lifetime of from two to twelve weeks. Thus, they are the immune system's way of directly addressing an antigen threat. When the threat is gone, the need for plasma cells is also gone. But, B lymphocytes remain, ready to differentiate into the antibody–producing plasma cells when required.

Within the past several years, research has indicated that the deliberate depletion of B cells might aid in thwarting the progression of autoimmune disease—where the body's immune system reacts against the body's own components—and so bring relief from, for example, rheumatoid arthritis. However, as yet the data is inconclusive, and so this promising therapy remains to be proven.

See also Antibody and antigen; Antibody formation and kinetics; Immunity, active, passive and delayed; Immunity, cell mediated; Immunization