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The cells of the immune system that make antibodies to invading pathogens like viruses. They form memory cells that remember the same pathogen for faster antibody production in future infections.
The cells of the immune system that make antibodies to invading pathogens like viruses. They form memory cells that remember the same pathogen for faster antibody production in future infections.

B cells are lymphocytes that play a large role in the humoral immune response (as opposed to the cell-mediated immune response, which is governed by T cells). A lymphocyte is a type of White blood cell in the Vertebrate Immune system. The Humoral Immune Response (HIR is the aspect of immunity that is mediated by secreted Antibodies (as opposed to Cell-mediated immunity which involves Cell-mediated immunity is an Immune response that does not involve Antibodies or complement but rather involves the activation of Macrophages T cells belong to a group of White blood cells known as Lymphocytes, and play a central role in Cell-mediated immunity. The principal function of B cells is to make antibodies against soluble antigens. Antibodies (also known as immunoglobulins, abbreviated Ig) are Gamma globulin Proteins that are found in Blood or other Bodily An antigen (from antibody-generating) or immunogen is a substance that prompts the generation of Antibodies and can cause an immune response B cells are an essential component of the adaptive immune system. Immune system|Passive immunity|Innate immune system The adaptive immune system is composed of highly specialized systemic cells and processes that eliminate or prevent Pathogenic

Contents

Development of B cells

Immature B cells are produced in the bone marrow of most mammals. Bone marrow is the flexible tissue found in the hollow interior of Bones In adults marrow in large bones produces new Blood cells It constitutes 4% of Rabbits are an exception; their B cells develop in the appendix-sacculus rotundus. Rabbits are small Mammals in the family Leporidae of the order Lagomorpha, found in several parts of the world After reaching the IgM+ immature stage in the bone marrow, these transitional B cells migrate to the spleen where they mature into B lymphocytes[1]. 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 fields of Genetics and Evolutionary computation, a locus (plural loci) is a fixed position on a Chromosome such as the position of a In the fields of Genetics and Evolutionary computation, a locus (plural loci) is a fixed position on a Chromosome such as the position of a 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. V(DJ recombination is a mechanism of Genetic recombination that occurs in Vertebrates which randomly selects and assembles segments of Genes encoding Antibodies (also known as immunoglobulins, abbreviated Ig) are Gamma globulin Proteins that are found in Blood or other Bodily 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, here called clonal deletion. Clonal deletion is a process by which B cells and T cells are deactivated before act significantly upon specific antigens [1] If it recognizes self-antigen during the maturation process, the B cell will become suppressed (known as anergy) or undergo apoptosis (also termed negative selection). B cells are continuously produced in the bone marrow, but only a small portion of newly made B cells survive to participate in the long-lived peripheral B cell pool.

B cell membrane receptors on which drugs may be active evolve during the B cell life span [2]. CD20 is present on preB cells, but disappears in mature B cells. TACI, BCMA and BAFF-R are present on immature B cells and mature B cells. The agonist of these 3 receptors is inhibited by Belimumab. Belimumab (registered name LymphoStat-B) is a fully human monoclonal antibody that specifically recognizes and inhibits the biological activity of B-Lymphocyte stimulator (BLyS

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 surveillance. Blood is a specialized Bodily fluid that delivers necessary substances to the body's cells €”such as nutrients and oxygen—and transports Waste products The lymphatic system in Vertebrates is a network of conduits that carry a clear fluid called Lymph. 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. Antibodies (also known as immunoglobulins, abbreviated Ig) are Gamma globulin Proteins that are found in Blood or other Bodily 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. Antibodies (also known as immunoglobulins, abbreviated Ig) are Gamma globulin Proteins that are found in Blood or other Bodily Once a B cell encounters its cognate antigen and receives an additional signal from a T helper cell, it can further differentiate into one of the two types of B cells listed below (plasma B cells and memory B cells). T helper cells (also known as effector T cells or Th cells) are a sub-group of Lymphocytes (a type of White blood cell or Plasma cells (also called plasma B cells or plasmocytes) are cells of the Immune system that secrete large amounts of antibodies. Memory B cells are a B cell sub-type that are formed following primary infection The B cell may either become one of these cell types directly or it may undergo an intermediate differentiation step, the germinal center reaction, where the B cell will hypermutate the variable region of its immunoglobulin gene ("somatic hypermutation") and possibly undergo class switching. Germinal centres (GC are areas within Lymph nodes where B lymphocytes rapidly divide and are an important part of the humoral immune response. Somatic hypermutation (or SHM is a mechanism inside cells that is part of the way the immune system adapts to the new foreign elements which confront it (for example Antibodies (also known as immunoglobulins, abbreviated Ig) are Gamma globulin Proteins that are found in Blood or other Bodily History See also History of genetics The existence of genes was first suggested by Gregor Mendel (1822-1884 who in the 1860s studied inheritance Immunoglobulin class switching (or isotype switching or isotypic commutation or class switch recombination) is a biological mechanism that changes

Clonality of B cells

B cell exist as clones, meaning, all the B cells derived from a particular cell, and thus, the antibodies their differentiated progenies (see below) produce can recognize and/or bind the same components (epitope) of a given antigen. A clone is a group of identical cells that share a common ancestry, meaning are derived from the same mother cell An epitope, also known as antigenic determinant, is the part of a Macromolecule that is recognized by the Immune system, specifically by antibodies This has important consequences, most notably, the phenomenon of immunogenic memory relies on this clonality of B cells. The great diversity in immune response comes about because there are up to 109 clones with that many specificities for recognizing antigens. A single B cell or a clone of cells with shared specificity upon encountering its specific antigen (rather, the epitope), divides many times to produce many B cells, most of which differentiate into Plasma cells that can secrete antibodies into blood that bind the same epitope that elicited proliferation in the first place, while a very small minority survive as memory cells that can again recognize only the same epitope, and when that happens would divide further to produce more plasma and memory cells. However, with each such cycle, the number of surviving memory cells increases. This also is accompanied by affinity maturation that increases the sum total of epitopes that can be recognized by the related clones through random mutations in the epitope binding (and recognizing) portions of these cell. This subsequent amplification with improved specificity of immune response is known as secondary immune response. If the cell that encountered the epitope to subsequently proliferate did so for the first time, it would be known as a naive cell, i. e. , neither it nor any of its predecessors ever encountered this epitope.

B cell types

A plasma cell
A plasma cell

Recognition of antigen by B cells

A critical difference between B cells and T cells is how each lymphocyte "sees" its antigen. T cells belong to a group of White blood cells known as Lymphocytes, and play a central role in Cell-mediated immunity. An antigen (from antibody-generating) or immunogen is a substance that prompts the generation of Antibodies and can cause an immune response 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 an antigen presenting cell's MHC molecule to the T cell receptor. Peptides (from the Greek πεπτίδια, "small digestibles" are short Polymers formed from the linking in a defined order of α- Amino See also Antigen presentation An antigen-presenting cell ( APC) or accessory cell is a cell that displays foreign Antigen complexed The major histocompatibility complex ( MHC) is a large genomic region or Gene family found in most Vertebrates It is the most gene-dense region The T cell receptor or TCR is a molecule found on the surface of T lymphocytes (or T cells that is in general responsible for recognizing Antigens bound

Mechanism of action.
Mechanism of action.

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). Plasma cells (also called plasma B cells or plasmocytes) are cells of the Immune system that secrete large amounts of antibodies. B cells that have not been exposed to antigen, also known as Naive B cells, can be activated in a T-cell dependent or independent manner.

T-cell dependent activation

When a macrophage ingests a pathogen, it attaches parts of the pathogen's proteins to a class II MHC protein. A pathogen (from Greek πάθος pathos "suffering passion" and γἰγνομαι (γεν- gignomai (gen- "I give birth to" infectious The major histocompatibility complex ( MHC) is a large genomic region or Gene family found in most Vertebrates It is the most gene-dense region This complex is moved to the outside of the cell membrane, where the epitope on the antigen can be recognized by a T cell, which is compatible with similar structures on the cell membrane of a B cell. An epitope, also known as antigenic determinant, is the part of a Macromolecule that is recognized by the Immune system, specifically by antibodies T cells belong to a group of White blood cells known as Lymphocytes, and play a central role in Cell-mediated immunity. If the B cell and T cell structures match, the T cell will activate the B cell, which starts producing antibodies in large scale against the bits of pathogen, called antigen, it has found with its B cell receptor (a membrane bound antibody). Since, many clones of B memory/naive cells can recognize the same antigen, the response is polyclonal.

Most antigens are T-dependent, meaning T cell help is required for maximal antibody production. With a T-dependent antigen, the first signal comes from antigen cross linking the B cell receptor (BCR) and the second signal comes from co-stimulation provided by a T cell. During the activation of T cells co-stimulation is often crucial to the development of an effective immune response. T cells belong to a group of White blood cells known as Lymphocytes, and play a central role in Cell-mediated immunity. T dependent antigens contain proteins that are presented on B cell Class II MHC to a special subtype of T cell called a Th2 cell. MHC ( Major histocompatibility complex) Class II molecules are found only on a few specialized cell types including Macrophages Dendritic cells and T helper cells (also known as effector T cells or Th cells) are a sub-group of Lymphocytes (a type of White blood cell or When a B cell processes and presents the same antigen to the primed Th cell, the T cell secretes cytokines that activate the B cell. Cytokines are a category of signalling Proteins and Glycoproteins that like Hormones and Neurotransmitters, are used extensively in cellular These cytokines trigger B cell proliferation and differentiation into plasma cells. Plasma cells (also called plasma B cells or plasmocytes) are cells of the Immune system that secrete large amounts of antibodies. Isotype switching to IgG, IgA, and IgE and memory cell generation occur in response to T-dependent antigens. Immunoglobulin G ( IgG) is a Monomeric Immunoglobulin, built of two heavy chains γ and two light chains. Immunoglobulin A ( IgA) is an Antibody playing a critical role in mucosal immunity In Biology, Immunoglobulin E ( IgE) is a class of Antibody (or Immunoglobulin " Isotype " that has only been found in This isotype switching is known as Class Switch Recombination (CSR). Once this switch has occurred, that particular B-cell can no longer make the earlier isotypes, IgM or IgD. Immunoglobulin M, or IgM for short is a basic Antibody that is present on B cells It is the primary antibody against A and B Immunoglobulin D ( IgD) is an Antibody isotype that makes up about 1% of Proteins in the Plasma membranes of immature B-lymphocytes

T-cell independent activation

Many antigens are T-independent, meaning they can deliver both of the signals to the B cell. Mice without a thymus (nude or athymic mice that do not produce any T cells) can respond to T-independent antigens. A mouse (plural mice) is a small Animal that belongs to one In Human anatomy, the thymus is an organ located in the upper Anterior portion of the chest cavity just behind the Sternum. Nudity is the state of wearing no Clothing. The term' "nudity" can also occasionally be used to refer to wearing significantly less clothing than expected Many bacteria have repeating carbohydrate epitopes that stimulate B cells, by cross-linking the IgM antigen receptors in the B-cell,responding with IgM synthesis in the absence of T cell help. An epitope, also known as antigenic determinant, is the part of a Macromolecule that is recognized by the Immune system, specifically by antibodies There are two types of T-cell independent activation; Type 1 T cell-independent (polyclonal) activation, and 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). Polyclonal antibodies (or antisera are antibodies that are derived from different B cell lines

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. Vertebrates are members of the Subphylum Vertebrata, Chordates with backbones or spinal columns The grouping sometimes includes Fish are aquatic Vertebrate animals that are typically ectothermic (previously Cold-blooded) covered with scales, and equipped with two Prehistoric amphibian Amphibians (class Amphibia such as Frogs Toads Salamanders Newts Gymnophiona, Sirens and Phagocytosis is the cellular process of engulfing solid particles by the Cell membrane to form an internal Phagosome, or "food vacuole Immune system|Adaptive immune systemThe innate immune system comprises the cells and mechanisms that defend the host from infection by other organisms in a non-specific manner 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[4]. Macrophages ( Greek: "big eaters" from makros "large" + phagein "eat" ( Mø) are cells within the tissues that A lymphocyte is a type of White blood cell in the Vertebrate Immune system. Immune system|Passive immunity|Innate immune system The adaptive immune system is composed of highly specialized systemic cells and processes that eliminate or prevent Pathogenic

B cells in humans (and other vertebrates) are nevertheless able to endocytose antibody-fixed pathogens, and it is through this route that MHC Class II presentation by B cells is possible, allowing Th2 help and stimulation of B cell proliferation. This is purely for the benefit of MHC Class II presentation, not as a significant method of reducing the pathogen load.

Origin of the word B cell

The abbreviation "B" in B cell originally came from Bursa of Fabricius, an organ in birds in which avian B cells mature. In Birds the bursa of Fabricius ( Latin: Bursa cloacalis or Bursa fabricii) is the site of Hematopoiesis, a specialized organ The cell is the structural and functional unit of all known living Organisms It is the smallest unit of an organism that is classified as living and is often called [5] When it was discovered that in most mammals immature B cells are formed in bone marrow, the word B cell continued to be used, although other blood cells also originate from pluripotent stem cells in the bone marrow. The fact that bone and bursa both start with the letter 'B' is a coincidence.

Additional image

Figure 1: Schematic diagram to explain mechanisms of clonal selection of B cell, and how secondary immune response is stronger, quicker and more specific in comparison with the primary one.
Figure 1: Schematic diagram to explain mechanisms of clonal selection of B cell, and how secondary immune response is stronger, quicker and more specific in comparison with the primary one. [6]

References

  1. ^ Parham, P. (2005). The Immune System, Garland Science Publishing, New York, NY.
  2. ^ Montecino-Rodriguez, Encarnacion'''''''; Kenneth Dorshkind (2006-09). "New perspectives in B-1 B cell development and function". Trends in Immunology' 27 (9): 428-433. Elsevier B. V. . doi:doi:[http://dx. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document. doi. org/10. 1016/j. it. 2006. 07. 005 10. 1016/j. it. 2006. 07. 005].  
  3. ^ [[James W Tung'''''''|Tung, James]]; Leonore A Herzenberg''''''' (2007-04). ""Unraveling B-1 progenitors"". Current Opinion in Immunology 19 (2): 150-155. Elsevier B. V. . doi:doi:[http://dx. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document. doi. org/10. 1016/j. coi. 2007. 02. 012 10. 1016/j. coi. 2007. 02. 012].  
  4. ^ J. Li, D. R. Barreda, Y. -A. Zhang, H. Boshra, A. E. Gelman, S. LaPatra, L. Tort & J. O. Sunyer (2006). "B lymphocytes from early vertebrates have potent phagocytic and microbicidal abilities". Nature Immunology 7: 1116–1124. doi:10.1038/ni1389. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document. PMID 16980980.  
  5. ^ Bursa
  6. ^ Goldsby, Richard; Kindt, TJ; Osborne, BA; Janis Kuby (2003). Immunology Fifth Edition. New York: W. H. Freeman and Company, 119-120. ISBN 0-07167-4947-5.  

See also

External links

In Immunology, affinity maturation is the process by which B-cells produce Antibodies with increased affinity for Antigen during the course of Antibodies (also known as immunoglobulins, abbreviated Ig) are Gamma globulin Proteins that are found in Blood or other Bodily The clonal selection theory has become a widely accepted model for how the Immune system responds to Infection and how certain types of B and T Lymphocytes Original antigenic sin (first described in 1960 by Thomas Francis Jr Medical Subject Headings ( MeSH) is a huge Controlled vocabulary (or metadata system for the purpose of indexing journal articles and books

Dictionary

B cell

-noun

  1. (immunology) A lymphocyte, derived from bone marrow, that produces antibodies and is responsible for the immune system.
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