The Major Histocompatibility Complex ( MHC ) is the system present in human is known as the Human Leukocyte Antigen ( HLA ) system. The MHC system consists of some little group of cistrons present on human chromosome 6. The same defence system present in mice is known as H-2 system. The agreement of cistrons is different in human and mice. These groups of cistrons can split into three groups.
Those cistrons which encodes for the glycopeptides from the surface of nucleated cells is Class I MHC. This category is more active against the antigenic peptides and nowadayss it to cytotoxic T-cells.
The group of cistron that encodes the glycopeptides from merely three cells like B-cells, Macrophages and Dendritic cells is known as the Class MHC II. The Class MHC II present antigenic peptides to Helper T-cells.
The 3rd group of cistrons known as the Class III MHC cistrons. The merchandise of this group is inflammatory cytokines and complement protein. These proteins are required for the inflammatory response of cells.
The cistrons related to Class I MHC are located near the telomeric terminal, Class II MHC cistrons are present on the centromeric terminal and the Class III cistrons can be found between the Class 1 MHC and Class II MHC.
The Class I MHC in homo is known as Classical category I cistrons and are extremely expressed among the human cells. These cistrons are present on A, B and C parts. In the same category some other cistrons are present this group of cistrons are known as the nonclassical category I cistrons. The Class II MHC cistrons are present on the DP, DR and DQ parts. The first two MHC categories encodes the molecule which have same construction and involve in the presentation and dispensation of antigen. The Class III MHC is different as it encodes for some complement factors C2, C4, BF, inflammatory cytokines, Heat daze protein and tumour mortification factors.
The Class I and II have bit similar construction and are membrane delimited glycoprotein. These groups form the stable composite with antigen derived peptides on T-cell surface for acknowledgment of antigen. The molecules of Class I MHC consist of ? concatenation associated with ?2 microglobulin molecule noncovalently. The ? concatenation is encoded by A, B and C part of cistrons and it is a transmembrane protein where as ?2 is encoded by the cistrons nowadays on the conserved part of another chromosome. In item ? concatenation is consist of ?1, ?2 and ?3 and each portion have the about 90 aminic acids. The other ?2 unit has non transmembrane protein and is associated with similar to ?3 unit. This association shapes the peptide binding cleft which is specific against specific antigen. Without the ?2 the antigen will non presented on surface of cell. The Class I MHC is thought as the immunoglobuline household because of the sequence similarities with immunoglobuline. The ?3 sphere of Class I MHC is active against the CD8 cells and is responsible for its fond regard.
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Figure1. Class I MHC molecules. 1 ) Intracellular proteins are degraded in proteasomes. 2 ) This debasement synthesis antigenic peptides. 3 ) Antigenic peptides are transported into RER.4 ) Class I MHC molecules attach antigenic peptides in RER and for complex.5 ) these composites are transported from the Golgi to the surface of cell for presentation of peptide to CD8+ T cells.
The Class II MHC consist of nonidentical glycoprotein spheres, ? and ? . These two ironss has non covalent bond between each other. These each sphere have two units, ?1 and ?2 and on the other side ?1 and ?2. The Class II is categorized in immunoglobuline superfamily because the sequence of ?2 and ?2 spheres is similar to immunoglobuline. These spheres besides have antigen binding cleft for antigen dispensation.
The T-cells have the ability to acknowledge the antigen particularly if the antigen peptides attach to the MHC cleft on the surface of cell. The complete procedure is known as the Antigen Processing and Presentation. This procedure works by determining the MHC-peptide composite and precede when the antigenic peptide degraded and fond regard of this debauched peptides with MHC molecules on T-cell surface for acknowledgment. The peptide fond regard with MHC I and MHCII occur in different Chamberss of cells.
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Figure2. Class II MHC molecules 1 ) Extracellular proteins are acquire enter into endosomes. 2 ) Lysozomes fuse with endosomes. 3 ) Degradation of endocytosed proteins in the cyst, to organize antigenic peptides. 4 ) Class II MHC molecules, DM and invariant concatenation fuses with the endolysozomal cyst and debasement of invariant concatenation to CLIP, omission of cartridge holder, and Class II MHC molecules attach antigenic peptides. 5 ) Class II MHC-antigenic peptide composites transportes to the surface of cell, for presentation to CD4+ T cells.
In the procedure of antigen riddance the immune system undergoes through two distinct tracts.
In the Cytosolic pathway the endogenous derived antigenic peptides attach to Class I MHC molecule. These proteins can be normal or foreign atom derived protein. In the endocytic tract the Class II MHC attach to exogenic antigen derived peptides. These antigens enter in the endosome by the procedure of phagocytosis and endocytosis.
The proteasomes produce the peptides for presentation. The proteolytic system degrades the intracellular protein into little peptides. The protein which go through the proteolysis have short protein ubiquitin bind to it on lysine residue to organize ubiquitin protein composite. This complex really degraded into short peptides by proteasome and transferred to rough endoplasmic Reticulum ( RER ) lms. This proteasome sense the composite in a status when ubiquitin is bind to protein. The Class I MHC is synthesized in the RER with TAP proteins. This TAP protein plays its function in transit of peptides into the RER. This mechanism of MHC molecule complex involve many stairss like calrecticulin, tapasin, calnexin and cheperones. The molecule binds to ? unit foremost is the calnexin and do it turn uping.
The antigen is phagocyte or endocyte by the macrophage. The antigen is degraded into the peptides by the action of lysosome or endosome. The antigen travel from early to late endosome and so to lysosome which run into the hydrolytic enzymes in combination with low pH. Within the endocytic compartment the MHC II attaches the peptides on the cleft.
The endocytic compartment returns to the cell surface and fuse with plasma membrane. In response to a mechanism the peptides present in the RER lms which is specific to Class MHC I inhibit its binding with Class MHC II. This mechanism is carried out by Ii CD74 invariant concatenation. The Ii CD74 attaches to peptide adhering cleft of MHC I and suppress its fond regard with MHC II. After this fond regard the MHC molecules are transported to Golgi from RER where proteolytic activities degrade the invariant concatenation ( Ii CD74 ) and organize the CLIP to adhere MHC II. Subsequently on the CLIP is removed by the proteolytic activity of HLA-DM. in this manner the binding of peptide make the molecule stable and travel to the cell surface for sensing of TH cells.