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MHC class I and MHC class II molecules
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Now, focusing on just T cells - they can only bind antigens, which are typically short peptides, when these antigens are displayed on a Major Histocompatibility Complex or MHC molecule which is sort of like a silver platter that is on the surface of a cell.
The MHC molecules are also called human leukocyte antigens and these proteins are encoded for by MHC genes, which are found on chromosome 6.
There are actually two groups of genes. One group of genes encode the MHC class I molecule, which is bound by the CD8 molecule on the surface of cytotoxic T cells.
MHC class I genes encode the proteins HLA-A, HLA-B, and HLA-C, which is easy to remember as for MHC 1 it is always 1 letter.
MHC class II genes encode the proteins HLA-DP, DQ, and DR which is also easy to remember because for MHC class 2 there are always 2 letters.
And these genes are called histocompatibility because they are really important in determining whether or not a transplant is compatible or gets rejected.
But their role isn’t just to wreak havoc on transplants! They’re critically important in making sure that T cells recognize and react to antigens.
And even though they’re called human leukocyte antigens, they’re not just found in leukocytes or white blood cells.
MHC class I molecules are found on the surface of all nucleated cells in the body. These molecules present peptides from within the cell to the immune system, and play an important role in alerting the immune system about cells infected by viruses. On the other hand, MHC class II molecules are found only on antigen-presenting cells, such as macrophages, dendritic cells, and B cells. These molecules present peptides from extracellular pathogens to the immune system.
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