Key Difference –MHCI vs II
In the context of immunity,Major Histocompatibility Complex(MHC) is an important molecule during the recognition ofantigens(foreign substances). They are considered to be a set of cell surface proteins which basically function to bind with foreign antigens to present them on either of theT celltypes;T helper cells (TH) or cytotoxic T cells (TC)通过T细胞受体。MHC class I and MHC class II are encoded by genes present in human leukocyte antigen (HLA) system. MHC molecules present on each cell surface display a particular fraction of a protein molecule calledepitope。This prevents the cells immunity system from targeting its own cells during the presentation of antigens which can be either self or non-self antigens.MHC class I molecules present antigens on theco–receptormolecules known asCD8which are situated on Tc cells, in contrast, MHC class II molecules present antigens on theco–receptorCD4which are situated on THcells.This is thekey differencebetween MHC Class I and MHC class II.
CONTENTS
1.Overview and Key Difference
2.What is MHC I
3.What is MHC II
4.Similarities Between MHC I and II
5.Side by Side Comparison – MHC I vs II in Tabular Form
6.Summary
What is MHC I?
MHC Class I molecules are present on cell surfaces of all nucleated cells and are one of the main two classes of MHC molecules. These molecules don’t occur in红细胞but are present inplatelets。MHC Class I molecules detect protein fragments from nonself proteins within the cell. These protein fragments are known as antigens. Nonself antigens detected by MHC I molecules are situated on Tc cells. Tc cells possess coreceptor molecules, CD8. MHC I molecules which present antigens on CD8 receptors that will initiate animmunological response。
Since thepeptidespresent on MHC Class I molecules are derived from cytosolic proteins, the antigen presentation pathway of these molecules is referred as endogenous (cytosolic) pathway. MHC Class I molecules are composed of two nonidentical chains, long alpha chain, and one short beta chain. They are encoded by human leukocyte antigen genes (HLA) HLA-A, HLA-B, and HLA-C. Alpha chain is coded on the locus of MHC in chromosome 6 and beta chain is encoded on chromosome 15.
MHC分子函数显示一个信使ing intracellular proteins to Tc cells to prevent immunological responses directed upon host’s own cells. When intracellular proteins degrade by the proteasome, peptide particles bind to MHC I molecules. These peptide particles are known as epitopes. The MHC Class I protein complex is presented into the external plasma membrane of the cell via theendoplasmic reticulum。Afterwards, the epitopes get bound on extracellular surfaces of MHC I molecules. Due to this process, Tc cells will not be activated in response to self-antigens. This is known as T cell tolerance (Central and peripheral tolerance). MHC Class I proteins are capable of presenting exogenous antigens derived from various pathogens. This is known as cross-presentation. During such conditions, when a foreign antigen is presented on Tc cells by MHC I molecules, immunological responses will be initiated.
What is MHC II?
MHC Class II molecules are expressed by a specialized type of cells known asantigen presenting cells(APC)。APC includemacrophages,B cells, anddendritic cells。When an MHC Class II molecule encounters an antigen, it takes up the antigen into the cell, processes it, and then a fraction of a molecule of the antigen (epitope) is presented on the surface of MHC Class II. The peptide particles are derived fromphagocytosiswhere the extracellular proteins are endocytosed and digested bylysosomes。消化肽粒子加载到the MHC Class II before they migrate onto the surface of the cell. The epitope presented on the cell surface could recognize and bind complementary particles known asparatope。A paratope could be a self or nonself antigen. MHC Class II molecules possess two identical alpha and beta chains, which are encoded by MHC locus of chromosome 6.
These molecules are encoded by the gene HLA-D. MHC Class II molecules present antigens to other cells of the immune system to initiate an immunological response with the help of THcells. THcells possess a co-receptor known as CD4. With the involvement of CD4 and T cell receptor, MHC Class II molecules activate the T cell and create an immunological response. The main function of MHC Class II molecules is to clear exogenous antigens present within the cell.
What are the Similarities Between MHC I and II?
- Both molecules are synthesized in the rough endoplasmic reticulum.
- Both MHC I and MHC II are encoded by genes present in HLA location.
- Both molecules are present on the surface of APC.
- Expression of genes in both molecules is co-dominant.
What is the Difference Between MHC I and II?
MHC I vs MHC II |
|
MHC I is one of the two primary classes of Major Histocompatibility Complex (MHC) molecules and are found on the cell surface of all nucleated cells. | MHC II is a class of Major Histocompatibility Complex (MHC) molecules normally found only onantigen-presenting cellssuch as dendric cells, some endothelial cells,thymic epithelial cells, and B cells. |
Structure | |
The MHC I molecule is composed of two nonidentical chains; long alpha chain and one short beta chain. | MHC II molecule is composed of alpha and beta chains which are roughly identical. |
Location | |
MHC I are found on the cell surfaces of all nucleated cells. | MHC II is found in antigen presenting cells (APC) which includes B cells, macrophages, and dendritic cells. |
Interaction with T cells | |
MHC I interacts mainly with cytotoxic T cells (Tc). | MHC II interacts with T helper cells (Th). |
Encoded genes | |
MHC I is encoded by genes HLA-A, HLA-B and HLA-C. | MHC II is encoded by HLA-D. |
Function | |
MHC I involves in the clearance of endogenous antigens. | MHC II involves in the clearance of exogenous antigens. |
Summary – MHCI vs II
MHC molecules are mainly of two types, Class I and Class II. They are considered to be a set of cell surface proteins which basically function to bind with foreign antigens that are deriving from invading pathogens. Later, MHC molecules present these antigens on either of the T cell types; T helper cells (TH) or cytotoxic T cells (TC) through the T cell receptor. MHC class I molecules are present on the cell surfaces of all nucleated cells and MHC Class II molecules present in antigen presenting cells (APC) which includes B cells, macrophages, and dendritic cells. Both molecules are synthesized in the rough endoplasmic reticulum and MHC I and MHC II are encoded by genes present in HLA location. This can be described as the difference between MHC I and MHC II.
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You can download PDF version of this article and use it for offline purposes as per citation note. Please download PDF version hereDifference Between MHC class I and II
Reference:
1.Janeway, Charles A, and Jr. “The major histocompatibility complex and its functions.” Immunobiology: The Immune System in Health and Disease. 5th edition., U.S. National Library of Medicine, 1 Jan. 1970.Available here
2.Wieczorek, Marek, et al. “Major Histocompatibility Complex (MHC) Class I and MHC Class II Proteins: Conformational Plasticity in Antigen Presentation.” Frontiers in Immunology, Frontiers Media S.A., 2017.Available here
Image Courtesy:
1.’MHC Class 1’By User atropos235 on en.wikipedia – Own work(CC BY 2.5)viaCommons Wikimedia
2.’MHC Class 2′ By A09231315 – Own work(CC BY-SA 3.0)viaCommons Wikimedia
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