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B cells are lymphocytes that play a large role in the humoral immune response as opposed to the cell-mediated immune response that is governed by T cells.The abbreviation "B" comes from bursa of Fabricius that is an organ in birds in which avian B cells mature. The principal function of B cells is to make antibodies against soluble antigens. B cells are an essential component of the adaptive immune system.
Development of B cells B cells are produced in the bone marrow of most mammals. Rabbits are an exception; their B cells develop in the appendix-sacculus rotundus. B cell development occurs through several stages, each stage representing a change in the genome content at the antibody loci. An antibody is composed of two light (L) and two heavy (H) chains, and the genes specifying them are found in the 'H' chain locus and the 'L' chain locus. In the H chain loci there are three regions, V, D and J, which recombine randomly, in a process called VDJ recombination, to produce a unique variable domain in the immunoglobulin of each individual B cell. Similar rearrangements occur for L chain locus except there are only two regions, namely V and J. The list below describes the process of immunoglobulin formation at the different stages of B cell development. When the B cell fails in any step of the maturation process, it will die by a mechanism called apoptosis. If it recognizes self-antigen during the maturation process, the B cell 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. Functions The human body makes millions of different types of B cells each day that circulate in the blood and lymph performing the role of immune surveillence. They do not produce antibodies until they become fully activated. Each B cell has a unique receptor protein (referred to as the B cell receptor (BCR)) on its surface that will bind to one particular antigen. The BCR is a membrane-bound immunoglobulin, and it is this molecule that allows the distinction of B cells from other types of lymphocyte, as well as being the main protein involved in B cell activation. 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 has two choices at this stage; it can either become one of these cell types directly or it can go through an intermediate differentiation step (the germinal center reaction) where the B cell will hypermutate the variable region of its immunoglobulin gene and possibly undergo class switching. B cell types Recognition of antigen by B cells A critical difference between B cells and T cells is how each lymphocyte "sees" its antigen. B cells recognize their cognate antigen in its native form. They recognize free (soluble) antigen in the blood or lymph using their BCR or membrane bound-immunoglobulin. In contrast, T cells recognize their cognate antigen in a processed form, as a peptide fragment presented by a MHC molecule to the T cell receptor. Activation of B cells B cell recognition of antigen is not the only element necessary for B cell activation (a combination of clonal proliferation and terminal differentiation into plasma cells). Activation also depends on one of three mechanisms: Type 1 T cell-independent (polyclonal) activation, type 2 T cell-independent activation (in which macrophages present several of the same antigen in a way that causes cross-linking of antibodies on the surface of B cells), and, T cell-dependent activation. During T cell-dependent activation, an antigen presenting cell (APC) presents a processed antigen to a helper T (Th) cell, priming it. When a B cell processes and presents the same antigen to the primed Th cell, the T cell releases cytokines that activate the B cell. The ancestral roots of B cells In an October 2006 issue of Nature Immunology, it was reported that certain B-cells of primitive vertebrates (like fish and amphibians) are capable of phagocytosis, a function usually associated with cells of the innate immune system. The authors of this article postulate that these phagocytic B-cells represent the ancestral history shared between macrophages and lymphocytes; B-cells may have evolved from macrophage-like cells during the formation of the adaptive immune system•. See also | ||||||||
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