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43-436: Antigens presented by MHC class II molecules are exogenous, originating from extracellular proteins rather than cytosolic and endogenous sources like those presented by MHC class I . The loading of a MHC class II molecule occurs by phagocytosis . Extracellular proteins are endocytosed into a phagosome , which subsequently fuses with a lysosome to create a phagolysosome . Within the phagolysosome, lysosomal enzymes degrade

86-455: A guanine exchange factor , loads ARL14/ARF7 with GTP. Subsequently, ARF7EP interacts with MYO1E which binds itself to actin myofibers. Altogether, this complex contributes to maintain MHC-II loaded vesicles within the immature dendritic cell , impeding its translocation to the cell membrane. One type of MHC class II deficiency, also called bare lymphocyte syndrome , is due to mutations in

129-438: A BCR binds a TD antigen, the antigen is taken up into the B cell through receptor-mediated endocytosis , degraded , and presented to T cells as peptide pieces in complex with MHC-II molecules on the cell membrane. T helper (T H ) cells , typically follicular T helper (T FH ) cells recognize and bind these MHC-II-peptide complexes through their T cell receptor (TCR) . Following TCR-MHC-II-peptide binding, T cells express

172-559: A common microbial constituent to toll-like receptors (TLRs) or by extensive crosslinking of BCRs to repeated epitopes on a bacterial cell. B cells activated by TI antigens go on to proliferate outside lymphoid follicles but still in SLOs (GCs do not form), possibly undergo immunoglobulin class switching, and differentiate into short-lived plasmablasts that produce early, weak antibodies mostly of class IgM, but also some populations of long-lived plasma cells. Memory B cell activation begins with

215-403: A different exon within the gene, and some genes have further domains that encode leader sequences, transmembrane sequences, etc. These molecules have both extracellular regions as well as a transmembrane sequence and a cytoplasmic tail. The α1 and β1 regions of the chains come together to make a membrane-distal peptide-binding domain, while the α2 and β2 regions, the remaining extracellular parts of

258-426: A full-force antibody immune response due to activation of B cells . During synthesis of class II MHC in the endoplasmic reticulum, the α and β chains are produced and complexed with a special polypeptide known as the invariant chain . The nascent MHC class II protein in the rough ER has its peptide-binding cleft blocked by the invariant chain (Ii; a trimer) to prevent it from binding cellular peptides or peptides from

301-443: A germinal center reaction where they generate plasma cells and more memory B cells. It is unclear whether the memory B cells undergo further affinity maturation within these secondary GCs. In vitro activation of memory B cells can be achieved through stimulation with various activators, such as pokeweed mitogen or anti- CD40 monoclonal antibodies , however, a study found a combination of R-848 and recombinant human IL-2 to be

344-614: A host of cancers , including chronic lymphocytic leukemia (CLL) , acute lymphoblastic leukemia (ALL) , hairy cell leukemia , follicular lymphoma , non-Hodgkin's lymphoma , Hodgkin's lymphoma , and plasma cell malignancies such as multiple myeloma , Waldenström's macroglobulinemia , and certain forms of amyloidosis . Abnormal B cells may be relatively large and some diseases include this in their names, such as diffuse large B-cell lymphomas (DLBCLs) and intravascular large B-cell lymphoma . Patients with B cell alymphocytosis are predisposed to infections. A study that investigated

387-535: A lymphoid follicle and forming a germinal center (GC) , which is a specialized microenvironment where B cells undergo extensive proliferation, immunoglobulin class switching, and affinity maturation directed by somatic hypermutation. These processes are facilitated by T FH and follicular dendritic cells within the GC and generate both high-affinity memory B cells and long-lived plasma cells. Resultant plasma cells secrete large numbers of antibodies and either stay within

430-467: A multitude of different diseases, one of which being Type I diabetes . HLA class II genes are the most important genes associated with the risk of inheriting Type I diabetes, accounting for about 40-50% of heritability . Alleles of these genes that affect peptide binding to the MHC class II molecules seem to impact Type I diabetes risk the most. Specific allele polymorphisms have been identified to increase

473-572: A net equilibrium of phosphorylation and non-phosphorylation. It is only when the cell comes in contact with an antigen presenting cell that the larger CD45 is displaced due to the close distance between the two membranes. This allows for net phosphorylation of the BCR and the initiation of the signal transduction pathway . Of the three B cell subsets, FO B cells preferentially undergo T cell-dependent activation while MZ B cells and B1 B cells preferentially undergo T cell-independent activation. B cell activation

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516-403: A plasmablast or plasma cell. In addition, B cells present antigens (they are also classified as professional antigen-presenting cells, APCs ) and secrete cytokines . In mammals B cells mature in the bone marrow , which is at the core of most bones . In birds , B cells mature in the bursa of Fabricius , a lymphoid organ where they were first discovered by Chang and Glick, which is why

559-583: A two-step differentiation process that yields both short-lived plasmablasts for immediate protection and long-lived plasma cells and memory B cells for persistent protection. The first step, known as the extrafollicular response, occurs outside lymphoid follicles but still in the SLO. During this step activated B cells proliferate, may undergo immunoglobulin class switching, and differentiate into plasmablasts that produce early, weak antibodies mostly of class IgM. The second step consists of activated B cells entering

602-442: A type of white blood cell of the lymphocyte subtype. They function in the humoral immunity component of the adaptive immune system . B cells produce antibody molecules which may be either secreted or inserted into the plasma membrane where they serve as a part of B-cell receptors . When a naïve or memory B cell is activated by an antigen, it proliferates and differentiates into an antibody-secreting effector cell, known as

645-413: Is a deficiency in MHC class II molecules B cells are not activated and cannot differentiate into plasma cells which causes them to be deficient in antibodies which are unable to perform as they are expected. The only current form of treatment is a bone-marrow transplant however even this does not cure the disease and most patients do not live past age ten. MHC class II genes and molecules are related to

688-432: Is also expressed on group 3 innate lymphoid cells . Having MHC class II molecules present proper peptides that are bound stably is essential for overall immune function. Because class II MHC is loaded with extracellular proteins, it is mainly concerned with presentation of extracellular pathogens (for example, bacteria that might be infecting a wound or the blood). Class II molecules interact mainly with immune cells, like

731-457: Is enhanced through the activity of CD21 , a surface receptor in complex with surface proteins CD19 and CD81 (all three are collectively known as the B cell coreceptor complex). When a BCR binds an antigen tagged with a fragment of the C3 complement protein, CD21 binds the C3 fragment, co-ligates with the bound BCR, and signals are transduced through CD19 and CD81 to lower the activation threshold of

774-525: Is then acquired and loaded onto a MHC II molecule. The MHC II molecule then travels to the surface to present the antigen to a helper T cell . MHC II activate helper T cells which help release cytokines and other things which will help induce other cells which help to combat the pathogens outside the cells. Several molecules are involved in this pathway. PIK3R2 and PIP5K1A are two kinases that phosphorylate Phosphatidylinositol (PIP) providing PSD4 with substrates for its GTP loading ability. PSD4 as

817-415: The B stands for bursa and not bone marrow , as commonly believed. B cells, unlike the other two classes of lymphocytes, T cells and natural killer cells , express B cell receptors (BCRs) on their cell membrane . BCRs allow the B cell to bind to a foreign antigen , against which it will initiate an antibody response. B cell receptors are extremely specific, with all BCRs on a B cell recognizing

860-499: The T helper cell ( CD4 ). The peptide presented regulates how T cells respond to an infection. Stable peptide binding is essential to prevent detachment and degradation of a peptide, which could occur without secure attachment to the MHC molecule. This would prevent T cell recognition of the antigen, T cell recruitment, and a proper immune response. The triggered appropriate immune response may include localized inflammation and swelling due to recruitment of phagocytes or may lead to

903-522: The plasma membrane by the APCs(antigen presenting cells). In some cells, antigens bind to recycled MHC class II molecules while they are in the early endosomes , while other cells such as dendritic cells internalize antigens via receptor-mediated endocytosis and create MHC class II molecules plus peptide in the endosomal-lysosomal antigen processing compartment which is independent of the synthesis of new MHC class II complexes. These suggest that after

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946-441: The B cell binds to an antigen via its BCR. Although the events taking place immediately after activation have yet to be completely determined, it is believed that B cells are activated in accordance with the kinetic segregation model , initially determined in T lymphocytes. This model denotes that before antigen stimulation, receptors diffuse through the membrane coming into contact with Lck and CD45 in equal frequency, rendering

989-416: The BCR and other receptors. Once differentiated, they are now considered mature B cells, or naïve B cells. B cell activation occurs in the secondary lymphoid organs (SLOs), such as the spleen and lymph nodes . After B cells mature in the bone marrow, they migrate through the blood to SLOs, which receive a constant supply of antigen through circulating lymph . At the SLO, B cell activation begins when

1032-508: The HLA gene complex can lead to bare lymphocyte syndrome (BLS), which is a type of MHC class II deficiency. Like MHC class I molecules, class II molecules are also heterodimers , but in this case consist of two homogenous peptides, an α and β chain, both of which are encoded in the MHC. The subdesignation α1, α2, etc. refers to separate domains within the HLA gene; each domain is usually encoded by

1075-724: The SLO or, more preferentially, migrate to bone marrow. Antigens that activate B cells without T cell help are known as T cell-independent (TI) antigens and include foreign polysaccharides and unmethylated CpG DNA. They are named as such because they are able to induce a humoral response in organisms that lack T cells. B cell response to these antigens is rapid, though antibodies generated tend to have lower affinity and are less functionally versatile than those generated from T cell-dependent activation. As with TD antigens, B cells activated by TI antigens need additional signals to complete activation, but instead of receiving them from T cells, they are provided either by recognition and binding of

1118-414: The antigen is internalized, already existent MHC class II complexes on mature dendritic cells can be recycled and developed into new MHC class II molecules plus peptide. Unlike MHC I, MHC II is meant to present extracellular pathogens rather than intracellular. Furthermore, the first step is to acquire the pathogen through phagocytosis. The pathogen is then broken down in a lysosome and a desired component

1161-469: The cell membrane. Memory T helper (T H ) cells, typically memory follicular T helper (T FH ) cells, that were derived from T cells activated with the same antigen recognize and bind these MHC-II-peptide complexes through their TCR. Following TCR-MHC-II-peptide binding and the relay of other signals from the memory T FH cell, the memory B cell is activated and differentiates either into plasmablasts and plasma cells via an extrafollicular response or enter

1204-460: The cell. Antigens that activate B cells with the help of T-cell are known as T cell-dependent (TD) antigens and include foreign proteins. They are named as such because they are unable to induce a humoral response in organisms that lack T cells. B cell responses to these antigens takes multiple days, though antibodies generated have a higher affinity and are more functionally versatile than those generated from T cell-independent activation. Once

1247-630: The chains, form a membrane-proximal immunoglobulin-like domain. The antigen binding groove, where the antigen or peptide binds, is made up of two α-helixes walls and β-sheet. Because the antigen-binding groove of MHC class II molecules is open at both ends while the corresponding groove on class I molecules is closed at each end, the antigens presented by MHC class II molecules are longer, generally between 15 and 24 amino acid residues long. These molecules are constitutively expressed in professional, immune antigen-presenting cells , but may also be induced on other cells by interferon γ . They are expressed on

1290-420: The detection and binding of their target antigen, which is shared by their parent B cell. Some memory B cells can be activated without T cell help, such as certain virus-specific memory B cells, but others need T cell help. Upon antigen binding, the memory B cell takes up the antigen through receptor-mediated endocytosis, degrades it, and presents it to T cells as peptide pieces in complex with MHC-II molecules on

1333-521: The endogenous pathway (such as those that would be loaded onto class I MHC). The invariant chain also facilitates the export of class II MHC from the ER to the Golgi apparatus , followed by fusion with a late endosome containing endocytosed, degraded proteins. The invariant chain is then broken down in stages by proteases called cathepsins , leaving only a small fragment known as CLIP which maintains blockage of

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1376-605: The epithelial cells in the thymus and on APCs in the periphery. MHC class II expression is closely regulated in APCs by CIITA , which is the MHC class II transactivator. CIITA is solely expressed on professional APCs; however, non-professional APCs can also regulate CIITA activity and MHC II expression. As mentioned interferon γ (IFN γ) triggers the expression of CIITA and is also responsible for converting monocytes which are MHC class II negative cells into functional APCs that express MHC class II on their surfaces. MHC class II

1419-458: The genes that code for transcription factors that regulate the expression of the MHC class II genes. It results in the depletion of CD4 T cells and some immunoglobulin isotypes even though there are normal levels of both CD8 Cells and B cells present. Deficient MHC class II molecules are unable to present antigens to T cells and properly activate T cells. T cells are then unable to proliferate, and secrete cytokines which normally participate in

1462-519: The immune response. Not only do the deficient MHC class II molecules affect the activation and proliferation of T cells but also the rest of the immune response cascade which includes B cells. Therefore, with this decrease in the number of T cells, the T cells cannot interact and activate the B cells. Normally when B cells are activated they divide, proliferate and differentiate, which includes the differentiation of these cells into plasma cells which are responsible for producing antibodies. However, when there

1505-429: The latter due to B cells undergoing V(D)J recombination as they develop. B cells undergo two types of selection while developing in the bone marrow to ensure proper development, both involving B cell receptors (BCR) on the surface of the cell. Positive selection occurs through antigen-independent signalling involving both the pre-BCR and the BCR. If these receptors do not bind to their ligand , B cells do not receive

1548-526: The mature B cells do not bind self antigens present in the bone marrow. To complete development, immature B cells migrate from the bone marrow into the spleen as transitional B cells , passing through two transitional stages: T1 and T2. Throughout their migration to the spleen and after spleen entry, they are considered T1 B cells. Within the spleen, T1 B cells transition to T2 B cells. T2 B cells differentiate into either follicular (FO) B cells or marginal zone (MZ) B cells depending on signals received through

1591-456: The most efficient activator. Autoimmune disease can result from abnormal B cell recognition of self-antigens followed by the production of autoantibodies. Autoimmune diseases where disease activity is correlated with B cell activity include scleroderma , multiple sclerosis , systemic lupus erythematosus , type 1 diabetes , post-infectious IBS , and rheumatoid arthritis . Malignant transformation of B cells and their precursors can cause

1634-401: The peptide binding cleft on the MHC molecule. A MHC class II-like structure, HLA-DM , facilitates CLIP removal and allows the binding of peptides with higher affinities. The stable class II MHC is then presented on the cell surface. After MHC class II complexes are synthesized and presented on APCs they are unable to be expressed on the cell surface indefinitely, due to the internalization of

1677-405: The proper signals and cease to develop. Negative selection occurs through the binding of self-antigen with the BCR; if the BCR can bind strongly to self-antigen, then the B cell undergoes one of four fates: clonal deletion , receptor editing , anergy , or ignorance (B cell ignores signal and continues development). This negative selection process leads to a state of central tolerance , in which

1720-546: The proteins into peptide fragments . These fragments are then loaded into the peptide-binding groove of the MHC class II molecule. Once loaded, the MHC class II-peptide complexes are transported to the plasma membrane via vesicular transport , where they present the antigens to the extracellular environment. In humans, the MHC class II protein complex is encoded by the human leukocyte antigen gene complex (HLA) . HLAs corresponding to MHC class II are HLA-DP , HLA-DM , HLA-DOA , HLA-DOB , HLA-DQ , and HLA-DR . Mutations in

1763-555: The risk (such as DRB1 and DQB1). Others have been associated with a resistance to the disease. Antigens Too Many Requests If you report this error to the Wikimedia System Administrators, please include the details below. Request from 172.68.168.236 via cp1112 cp1112, Varnish XID 976884681 Upstream caches: cp1112 int Error: 429, Too Many Requests at Thu, 28 Nov 2024 10:54:02 GMT B cell B cells , also known as B lymphocytes , are

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1806-450: The same epitope . B cells develop from hematopoietic stem cells (HSCs) that originate from bone marrow . HSCs first differentiate into multipotent progenitor (MPP) cells, then common lymphoid progenitor (CLP) cells. From here, their development into B cells occurs in several stages (shown in image to the right), each marked by various gene expression patterns and immunoglobulin H chain and L chain gene loci arrangements,

1849-704: The surface protein CD40L as well as cytokines such as IL-4 and IL-21 . CD40L serves as a necessary co-stimulatory factor for B cell activation by binding the B cell surface receptor CD40 , which promotes B cell proliferation , immunoglobulin class switching , and somatic hypermutation as well as sustains T cell growth and differentiation. T cell-derived cytokines bound by B cell cytokine receptors also promote B cell proliferation, immunoglobulin class switching, and somatic hypermutation as well as guide differentiation. After B cells receive these signals, they are considered activated. Once activated, B cells participate in

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