Difference Between MHC Class I And MHC Class II Proteins: Definition, Differences & Roles

MHC is also known as Major Histocompatibility Complex is a closely linked set of polymorphic genes that are coded for cell surface proteins which are highly essential for the adaptive immune system. This was discovered during Snell's experiments on transplantation. When the skin of blood group II containing mice were transplanted to some other blood group II containing mice, the skin was accepted, whereas on transplanting the skin of blood group II containing mice to some other blood group III containing mice, the skin was rejected. These MHC proteins were thus known to be very similar among all species but yet different or foreign enough to be recognized by an animal of different genetic backgrounds. Let’s discuss the difference between MHC class I and MHC class II.

Key Takeaways: MHC proteins, MHC Class I protein, MHC Class II protein, Proteins, Blood group, Genes, Blood, T-cells

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What is MHC?

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MHC is known as the Major Histocompatibility Complex which is a tightly linked clustered set of genes that code for proteins and are found on the surfaces of cells and help the adaptive body immune system to recognize any foreign matter. They are also required to present antigen for recognition by the appropriate T-cells. 

Major Histocompatibility Complex

In humans, MHC or Major Histocompatibility Complex is known as Human Lymphocyte Antigen(HLA) Complex. Tissue rejection due to incompatibility masks the real function of the MHC molecules The real function of an MHC protein is to bind an antigen derived from self-proteins or pathogens and bring the antigen expression to the cell surface for recognition by the appropriate T-cells. 

Also read: Proteins

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MHC Protein Types

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MHC Proteins are classified into various types, two of them being the most prevalent-

• MHC Class I

MHC Class I molecules are one of the two primary classes of Major Histocompatibility Complex (MHC) molecules and are found on the cell surface of all nucleated cells in the bodies of all living beings or vertebrates. They also occur on platelets but are absent on red blood cells that lack nuclei. Their function is to display peptide fragments of proteins from within the cell to T-cells. This will trigger an immediate response from the immune system against a particular antigen presented by an MHC Class I protein. Because MHC Class I molecules present in peptides are derived from cytosolic proteins, the pathway of MHC Class I presentation is often called a cytosolic or endogenous pathway.

Differences between MHC Class I and MHC Class II Proteins

• MHC Class II

MHC Class II proteins belong to a primary class of MHC molecules and are present on antigen-presenting cells like dendritic cells, endothelial cells, B cells, thymic epithelial cells, and mononuclear phagocytes. These cells initiate immune responses and therefore, are crucial. Unlike MHC Class I antigens the peptides are not cytosolic, the antigens presented in MHC Class II peptides are derived from extracellular proteins. In humans, the MHC Class II protein complex is encoded by the Human Leukocyte Antigen(HLA) gene complex.

Also read: Nucleic acids

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Differences between MHC Class I and MHC Class II Proteins

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The major differences between MHC Class I and MHC Class II proteins are follows:

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Similarities Between MHC Class I vs. MHC Class II Proteins

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1. Both the proteins are synthesized in the rough endoplasmic reticulum.
2. They are found on the surface of APC surface.
3. The expression of genes is co-dominant.
4. These are encoded by genes in the HLA locus.

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Things to Remember 

• MHC is also known as Major Histocompatibility Complex which is a closely linked set of polymorphic genes that are coded for cell surface proteins and are very essential for the adaptive immune system. It was discovered in an experiment for tissue rejection during transplantation.
• In humans, the MHC or Major Histocompatibility Complex is known as the Human Lymphocyte Antigen(HLA) Gene Complex. In mice, the Major Histocompatibility Complex is known as the H2 complex.
• MHC Proteins are classified into two primary classes, two of them being- MHC Class I and MHC Class II.
• MHC Class I molecules are one of the two primary classes of Major Histocompatibility Complex (MHC) molecules and are found on the cell surface of all nucleated cells in the bodies of all living beings or vertebrates. They also occur on platelets but are absent on red blood cells that lack nuclei. Their function is to display peptide fragments of proteins from within the cell to T-cells.
• The function of MHC Class I molecules is to display peptide fragments of proteins from within the cell to T-cells. This will trigger an immediate response from the immune system against a particular antigen presented by an MHC Class I protein.
• MHC Class II proteins belong to a primary class of MHC molecules and are present on antigen-presenting cells like dendritic cells, endothelial cells, B cells, thymic epithelial cells, and mononuclear phagocytes.