Comparison of Structure and Function of MHC Class I and Class II Proteins
MHC is nothing but a Major Histocompatibility Complex. These are a group of genes that code for proteins. These are found on the surface of the cells which help the body’s immune system recognize any foreign matter. It got this name since it was discovered during a study of tissue compatibility at the time of transplantation. Thus we can safely say that MHC checks upon the compatibility of the donor during the process of organ transplantation. It can also determine the chances of getting the autoimmune disease through cross-reacting immunization.
MHC proteins are present in all the higher vertebrates. MHC protein is of two types and they are:
MHC Class I
MHC Class II
MHC Class I
The MHC class I molecules are present in almost every nucleated cell in a living being. These are present in platelets as well but are absent in red blood cells that lack nuclei. The MHC class I protein helps in building cellular immunity. This immunity is necessary to take care of the pathogens that are capable of growing and reproducing inside the cells of their hosts. In 1989, the first structure of the MHC class I molecule was published named human HLA-A2. The main function of the class I MHC is to present the proteins that lie inside the cell to cytotoxic T cells which are also called ( CTLs ).
During the time of any infection, for example, consider a viral infection, the cell happens to release a foreign protein. At this time, as part of the MHC class I will exhibit these peptides on the cell surface. As a result, the CTLs that are specific for the MHC peptide complex will find and kill the presenting cells.
MHC Class II
Unlike MHC class I, the MHC class II molecules are confined to macrophages and lymphocytes which are the cells of the immune system. These are mainly found on dendritic cells, B cells, macrophages, etc.
When it comes to genes, each gene in our body has a large number of alleles. Alleles are nothing but the alternate form of genes that are capable of producing alternate forms of proteins. Therefore, we can easily conclude that it’s a very rare chance that any two individuals can have a similar set of MHC molecules. The MHC also contains a number of other genes that code for other proteins. These are called the class III MHC molecules.
During the early 1950s, when the skin graft experiments used to be carried out in mice, there were graft rejections. And these graft rejections were concluded saying that it was immune reactions by the host organism against the foreign matter or tissue. The MHC molecules on the cells of the graft tissue were recognized by the host as the foreign antigen. Therefore, for successful organ transplantation, it was necessary that the tissue type of the donor and the receiver should be similar to a large extent.
Difference Between MHC Class I and MHC Class II Proteins
FAQs on Difference Between MHC Class I and Class II Proteins in Immunity
1. What is the difference between MHC Class I and MHC Class II proteins?
The main difference between MHC Class I and MHC Class II proteins is that Class I presents antigens to CD8+ cytotoxic T cells, while Class II presents antigens to CD4+ helper T cells.
- MHC Class I: Found on almost all nucleated cells; presents intracellular (endogenous) antigens.
- MHC Class II: Found mainly on antigen-presenting cells; presents extracellular (exogenous) antigens.
- Class I triggers killing of infected cells; Class II activates immune coordination.
2. What is the function of MHC Class I proteins?
MHC Class I proteins present intracellular antigen fragments to CD8+ cytotoxic T lymphocytes to eliminate infected or abnormal cells.
- Display peptides from viruses or mutated proteins.
- Activate cytotoxic T cells.
- Lead to destruction of infected or cancerous cells.
3. What is the function of MHC Class II proteins?
MHC Class II proteins present extracellular antigen fragments to CD4+ helper T cells to coordinate immune responses.
- Found on antigen-presenting cells like dendritic cells, macrophages, and B cells.
- Activate helper T cells.
- Stimulate B cells, macrophages, and other immune cells.
4. Where are MHC Class I and Class II molecules found in the body?
MHC Class I molecules are present on all nucleated cells, while MHC Class II molecules are restricted to professional antigen-presenting cells.
- Class I: Found on nearly every body cell except red blood cells.
- Class II: Found on dendritic cells, macrophages, and B lymphocytes.
5. How do MHC Class I and Class II process antigens differently?
MHC Class I processes endogenous antigens in the cytoplasm, while MHC Class II processes exogenous antigens inside vesicles.
- Class I pathway:
- Proteins degraded by proteasomes.
- Peptides transported into the endoplasmic reticulum.
- Loaded onto MHC I and transported to the cell surface.
- Class II pathway:
- Extracellular antigens engulfed by endocytosis or phagocytosis.
- Processed in lysosomes.
- Loaded onto MHC II and displayed on the surface.
6. What types of T cells recognize MHC Class I and Class II?
MHC Class I is recognized by CD8+ cytotoxic T cells, whereas MHC Class II is recognized by CD4+ helper T cells.
- CD8+ T cells kill infected or abnormal cells.
- CD4+ T cells regulate and enhance immune responses.
7. What are examples of MHC Class I and MHC Class II genes?
MHC Class I genes include HLA-A, HLA-B, and HLA-C, while MHC Class II genes include HLA-DP, HLA-DQ, and HLA-DR.
- Located in the human leukocyte antigen (HLA) region on chromosome 6.
- Highly polymorphic, increasing immune diversity.
8. Why are MHC molecules important in organ transplantation?
MHC molecules are important in transplantation because mismatched MHC proteins can trigger immune rejection.
- Recipient T cells recognize donor MHC as foreign.
- Leads to activation of immune attack.
- HLA matching reduces rejection risk.
9. What is the structural difference between MHC Class I and Class II proteins?
MHC Class I consists of one alpha chain and a β2-microglobulin, while MHC Class II consists of two transmembrane chains (alpha and beta).
- Class I: Peptide-binding groove formed by a single alpha chain.
- Class II: Groove formed by both alpha and beta chains.
- Class II groove is open at both ends, allowing longer peptides.
10. What happens if MHC Class I or Class II proteins are defective?
Defective MHC Class I or Class II proteins can cause impaired immune responses and immunodeficiency disorders.
- Class I deficiency may reduce cytotoxic T cell activity.
- Class II deficiency (Bare Lymphocyte Syndrome) impairs helper T cell activation.
- Leads to recurrent infections and weak adaptive immunity.
