B cell
B cells are
lymphocytes that play a large role in the
humoral immune response (as opposed to the
cell-mediated immune response).The abbreviation "B" stands for the
bursa of Fabricius which is an organ unique to birds, where avian B
cells mature. It does
not stand for
bone marrow, where B cells are produced in all other vertebrates except for
rabbits (where B cells develop in the
appendix-sacculus rotundus).
The human body makes millions of different types of B cells each day, and each type has a unique receptor protein (referred to as the
B cell receptor (BCR)) on its membrane that will bind to one particular antigen; at any one time in the human body millions of B cells are circulating in the
blood and
lymph, but are not producing antibodies.Once a B cell encounters its cognate antigen and receives an additional signal from a
helper T cell, it can further differentiate into one of the two types of B cells listed below. The B cell can either directly become one of these cell types or go through an intermediate differentiation step - the
germinal center reaction where the B cell will hypermutate the variable region of the antibody and possibly class switch.
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Plasma B cells secrete
antibodies which assist in the destruction of
antigens by binding to them and making them easier targets for
phagocytes (a process known as
opsonization). Antibodies can also elicit the complement cascade, which also makes antigens an easier target for other cells.
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Memory B cells are formed specific to the antigen(s) encountered during the primary immune response; able to live for a long time, these cells can respond quickly upon second exposure to the antigen for which they are specific.
Humoral immunity (the creation of antibodies that circulate in
blood plasma and
lymph) involves B cell
activation. Cell activation can be gauged using the
ELISPOT technique, which can determine the percentage of B cells that secrete any particular antibody.
B cells are characterised immunohistochemically in humans by the presence of
CD20 on the cell membrane. In mice,
CD45 (B220) is often used.
A critical difference between B cells and
T cells is how each cell "sees" an
antigen. B cells recognize their cognate antigen in its native form. In contrast, T cells recognize their cognate antigen in a processed form - as a
peptide in the context of an
MHC molecule.
Susumu Tonegawa won the
1987 Nobel Prize in Physiology or Medicine for demonstrating how B cells create the enormous diversity of antibodies from only a few genes. The Nobel presentation [
1] gives an overview.
B cells are produced through several stages, each stage represents a change in the genome content, in which the variety of antibodies are produced. The human antibody is composed of two light and two heavy chains, and there are genes specifying them, which is known as the 'H' chain loci and the 'L' chain loci. In the H chain loci there are three regions, V, D and J, and combinations of threes are drawn, in terms of rearrangement which results in deletions of bases between the two selected points, and results in formation of a unique combination. In the L chain loci there are only two regions, namely V and J, which undergoes the same process.
* Progenitor B cells - Contains Germline H genes, Germline L genes
* Early Pro-B cells - undergoes D-J rearrangement on the H chains
* Late Pro-B cells - undergoes V-DJ rearrangement on the H chains
* Large Pre-B cells - the H chain is VDJ rearranged, Germline L genes
* Small Pre-B cells - undergoes V-J rearrangement on the L chains
* Immature B cells - VJ rearranged on L chains, VDJ rearranged on H chains. There is start of expression of IgM receptors.
* Mature B cells - There is start of expression of IgD
When the B cells fails in any step of the maturation process, it will undergo apoptosis, and if it recognizes self-antigen during the maturation process, it will become suppressed (known as anergy) or undergo apoptosis.
B cells are continuously produced in the bone marrow, but only a small portion of newly made B cells survive to participate a part in the long-lived peripheral B cell pool.
*
T lymphocyte*
Antibody*
