MHC class I and MHC class II molecules

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MHC class I and MHC class II molecules


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USMLE® Step 1 questions

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High Yield Notes

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MHC class I and MHC class II molecules

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USMLE® Step 1 style questions USMLE

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A biochemist isolates a protein that is expressed on antigen-presenting cells. The protein is composed of two equal-length chains and encoded by the MHC gene's DP loci. Which of the following best characterizes this protein’s function?  


Your immune system has an innate immune response which is immediate and nonspecific, as well as an adaptive immune response which is delayed and specific.

The adaptive immune is carried out by lots of unique B cells and T cells, which are highly specific for pathogens based on their unique parts - called antigens.

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.

Another group of genes encode the MHC class II molecules which is bound by the CD4 molecule on the surface of helper 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.

HLA proteins that code for MHC class I molecules are found on all nucleated cells throughout the body, even platelets which are fragments of nucleated 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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