What is the difference between MHC and HLA molecules?

MHC” stands for “major histocompatibility complex,” while “HLA” is the short version of “human leukocyte antigen.” ... The main difference between the two groups is that MHC is often found in vertebrates, while HLA is only found in humans. To simplify, HLA is the human body's version of MHC. (www.differencebetween.net/science/biology-science/difference-between-mhc-and-hla/)

Thank you very much for you reply back Unfortunately, I think my book contradicts this as it talks about the MHC in pathogenesis in the human body so I am confused.

As above, MHC stands for major histocompatability complex while HLA stands for human leucocyte antigen. MHC is a molecule found on the surface of cells - all nucleated cells express MHC I molecules while only professional antigen presenting cells (macrophages, dendritic cells, B cells) express MHC II molecules on their surface. HLA usually refers to the genes which code for various types of proteins (which are subtypes of MHC molecules). HLA-A, HLA-B and HLA-C code for different MHC I molecules while HLA-DR, HLA-DQ and HLA-DP genes code for different types of MHC II molecules.

In summary, MHC refers to the broad categories of proteins (MHC I or MHC II) expressed on the surface of the cell, whereas HLA normally refers to the genes which code for these proteins, although they can also refer to the proteins expressed also. However, they are often used interchangeable. And you can use both terms in humans.

Thank you very much!!

To add, the reason your textbook uses MHC in the human context is because the strictness in interchangeability between MHC and HLA depends on the field.

In general immunology, MHC is predominantly used as research may be looking at murine, primate or human cells. Human MHC is called HLA. Mouse MHC is called H-2. Referring to the different species-specific gene/protein names constantly would be too confusing, so everything is MHC to make things easier and convey that the principles are similar regardless of species. Textbooks will also predominantly use MHC since they may be discussing human biology one paragraph and a mouse model the next.

In clinical science (i.e. providing a medical service and not working in general research) on the other hand, it's a cardinal sin to refer to "MHC". Medical treatment is only concerning humans, so HLA is always used. The only time MHC is ever used in human biology is the "MHC Gene Region", the portion of Chr6 where HLA genes reside.

Ah thanks so much for this!
Could you please kindly check this for me if possible?

So if we are talking about just humans, we can say we have MHC genes or we can say we have HLA genes? Edit: Just saw your edit so changed my mind.. How about this: In humans, we have HLA genes and HLA genes are only found in the MHC gene region of chromosome 6 and nowhere else.

However..

When a macrophage has digested a pathogen, it combines antigens from the pathogen's surface membrane with special glycoproteins in the cytoplasm called the human leucocyte antigen complex ( I don't think this sounds right but we can't use MHC complex here because we are referring to humans). The HLA complex moves these pathogen antigens to the macrophage's own surface membrane, becoming an antigen-presenting cell (APC).

Thanks so much in advance!

When discussing human biology HLA is both the name of the gene and the protein. As such, it's important that you are clear whether you're referring to the HLA gene or the HLA protein. As you've done in your example the easiest way to be clear when referring to the protein is to write the "HLA complex", "HLA protein", "HLA gene-product". Just whatever you can in each sentence where HLA is used to make it clear what you're referring to. Immunology courses will likely be quite lenient on this though so don't be too worried. Whether you're referring to the gene or protein is often blatantly clear from the context of the sentence. Just be mindful of this if you ever flip from referring to the gene to referring to the protein (or vice versa) in quick succession.

The other way of discerning from the gene/protein is through molecular (i.e. genetic) nomenclature or serological (i.e. protein) nomenclature. For example, HLA-A*02 is the molecular format for describing the gene, and HLA-A2 is the serological format for referring to the protein. It gets a bit more complicated than this, but chances are you won't be using these formats in a generalised essay to describe principles! These are used more to describe specific cases when the allele or antigen group is important.

Thank you so much!!

No worries, HLA is my jam!

No worries, HLA is my jam!

Hi,

Since we have genes for different HLA antigens on chromosome 6, don't we have different HLA antigens covering the surface of the cells in our body?

However, the above wouldnt make sense either as if that was the case, then we wouldnt really be having issues with organ transplants.

Could you tell me what you think about this please, whenever you have the chance to do so?

Great question! I work in transplantation, so happy to help.

Yes, there are different HLA. Classical HLA can be broadly categorised into two groups: class I (A, B, C) and class II (DR, DQ, DP). Class I is found on the surface of all nucleated cells (so pretty much everything except red blood cells). Class II is found on antigen presenting cells (dendritic cells, macrophages, b cells). This is the general principle, though there are some exceptions for cells at certain states - endothelial cells can present HLA class II when activated for example, which has implications for organ transplantation.

When it comes to individual HLA gene/proteins, expression isn't equal among all cells. For example, platelets only express HLA class I, but they mainly express HLA-A and B. HLA-C expression is very weak on platelets, so not considered when selecting HLA matched platelets. In general C and DP expression seems to be fairly low compared to the other genes. There is a theory that HLA C expression is different to A and B because HLA-A and B evolved for interaction with T cells, while HLA-C evolved for interaction with Natural Killer Cells (but that's another story). DP isn't really considered in transplantation unless HLA-specific antibodies are present. DP is coming back into fashion for stem cell transplants, but it's more of a 'cherry on top' detail for better results rather than be all and end all major decider of success.

In terms of transplantation, the relative importance of the genes is debatable. Both class I and class II are significant for tissues as the host immune system can recognise mismatches in either system. You wouldn't think class II would be much of an issue, but the endothelial expression coupled with passenger APCs mean it's still relevant. Passenger cells are blood cells inside the organ that can then leave and go into the host blood system. The organ is perfused (washed out) prior to transplantation to remove residual blood cells in the organ's blood vessels, but there will always be some passenger cells. This is because cells can enter the organ tissue, and there are also theories of unidentified sites for the immune system within organs. The big issue with passenger APCs is that they have processed donor antigens and will want to present them to host T cells. So they could encourage rejection, in theory. The overall influence of passenger APCs in the risk of rejection isn't known though.