page_title: B-cell receptor
section_title: Development and structure of the B cell Receptor
hierarchical_section_title: B-cell receptor / Development and structure of the B cell Receptor
caption_attribution_description: Name: Biology ID: 185cbf87-c72e-48f5-b51e-f14f21b5eabd@9.17 Language: English Summary: Biology is designed for multi-semester biology courses for science majors. It is grounded on an evolutionary basis and includes exciting features that highlight careers in the biological sciences and everyday applications of the concepts at hand. To meet the needs of today’s instructors and students, some content has been strategically condensed while maintaining the overall scope and coverage of traditional texts for this course. Instructors can customize the book, adapting it to the approach that works best in their classroom. Biology also includes an innovative art program that incorporates critical thinking and clicker questions to help students understand—and apply—key concepts. Subjects: Science and Technology Keywords: cells,DNA,gene,expressio,nphylogeny,conservation,biodiversity,biology,chemistry,evolution,proteins,bacteria,fungi,cell,membranes,inheritance,physiology,ecology,genetics,biotechnology,biosphere,photosynthesis,biological,molecules,cell structure,metabolismc,ellular respiration,genes,cell communication,cell reproduction,meiosis,sexual reproduction,genomics,study of life,cell function,animal diversity,animal reproduction,plant nutrition,soilseedless plantsplant physiologyplant reproductionanimal bodycirculatory systemdigestive systemendocrine systemnervous systemrespiratory systemorigin of speciesecosystemsarchaeasensory systemosmotic regulationimmune systemvertebratesviruspopulation ecologymusculoskeletal systembehavioral biologyanimal biologyanimal physiologybiology for majorscollege biologyeukaryotesinvertebratesmacromoleculesplant biologypopulation evolutionprokaryotesprotistsseed plants Print Style: CCAP Biology License: Creative Commons Attribution License (by 4.0) Authors: OpenStax Copyright Holders: Rice University Publishers: OpenStax OpenStax Biology Latest Version: 10.53 First Publication Date: Aug 22, 2012 Latest Revision: May 27, 2016 Last Edited By: OpenStax Biology
context_page_description: The B cell receptor is a transmembrane protein on the surface of a B cell. B cell receptors are composed of immunoglobulin molecules that form a type 1 transmembrane receptor protein, and are typical located on the outer surface of these lymphocyte cells. Through biochemical signaling and by physically acquiring antigens from the immune synapses, the BCR controls the activation of the B cell. B cells are able to gather and grab antigens by engaging biochemical modules for receptor clustering, cell spreading, generation of pulling forces, and receptor transport, which eventually culminates in endocytosis and antigen presentation. B cells’ mechanical activity adheres to a pattern of negative and positive feedbacks that regulate the quantity of removed antigen by manipulating the dynamic of BCR-antigen bonds directly. Particularly, grouping and spreading increase the relation of antigen with BCR, thereby proving sensitivity and amplification. On the other hand, pulling forces delinks the antigen from the BCR, thus testing the quality of antigen binding.
context_section_description: The first checkpoint in the development of a B cell is the production of a functional pre-BCR, which is composed of two surrogate light chains and two immunoglobulin heavy chains, which are normally linked to Ig-α and Ig-βsignaling molecules. Each B cell, produced in the bone marrow, is highly specific to an antigen. The BCR can be found in a number of identical copies of membrane proteins that are exposed at the cell surface.
  The B cell receptor is composed of two parts:
  A membrane-bound immunoglobulin molecule of one isotype (IgD, IgM, IgA, IgG, or IgE). With the exception of the presence of an integral membrane domain, these are identical to a monomeric version of their secreted forms.
  Signal transduction moiety: A heterodimer called Ig-α/Ig-β (CD79), bound together by disulfide bridges. Each member of the dimer spans the plasma membrane and has a cytoplasmic tail bearing an immunoreceptor tyrosine-based activation motif (ITAM).
  More analytically, the BCR complex consists of an antigen-binding subunit known as the membrane immunoglobulin (mIg), which is composed of two immunoglobulin light chains (IgLs) and two immunoglobulin heavy chains (IgHs) as well as two heterodimer subunits of Ig-α and Ig-β. In order for mIgM molecules to transport to the surface of the cell, there must be a combination of  Ig-α and Ig-β with the mIgM molecules. Pre-B cells that do not generate any Ig molecule normally carry both Ig-α and Ig-β to the cell surface.
  Heterodimers may exist in the B cells as either an association or combination with another pre B cell-specific proteins or alone, thereby replacing the mIgM molecule. Within the BCR, the part that recognizes antigens is composed of three distinct genetic regions, referred to as V, D, and J. All these regions are recombined and spliced at the genetic level in a combinatorial process that is exceptional to the immune system. There are a number of genes that encode each of these regions in the genome and can be joined in various ways to generate a wide range of receptor molecules. The production of this variety is crucial since the body may encounter many more antigens than the available genes. Through this process, the body finds a way of producing multiple different combinations of antigen-recognizing receptor molecules. Heavy chain rearrangement of the BCR entails the initial steps in the development of B cell. The short JH (joining) and DH (diversity) regions are recombined first in early pro-B cells in a process that is dependent on the enzymes RAG2 and RAG1. After the recombination of the D and J regions, the cell is now referred to as a “late pro-B” cell and the short DJ region can now be recombined with a longer segment of the VH gene.
  BCRs have distinctive binding sites that rely on the complementarity of the surface of the epitope and the surface of the receptor, which often occurs by non-covalent forces. Mature B-cells can only survive in the peripheral circulation for a limited time in when there is no specific antigen. This is because when cells do not meet any antigen within this time, they will go through apoptosis. It is notable that in the peripheral circulation, apoptosis is important in maintaining an optimal circulation of B-lymphocytes. In structure, the BCR for antigens are almost identical to secreted antibodies. However, there is a distinctive structural dissimilarity in the C-terminal area of the heavy chains, as it consists of a hydrophobic stretch that is short, which spreads across the lipid bilayer of the membrane.
caption: The general structure of the B-cell receptor includes a membrane-bound immunoglobulin molecule and a signal transduction region. Disulfide bridges connect the immunoglobulin isotype and the signal transduction region.
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