Types of MHC Molecules
There are two main classes of MHC molecules: 1.
Class I MHC: Found on almost all nucleated cells, these molecules present peptides derived from intracellular proteins to
CD8+ T cells.
2.
Class II MHC: Located primarily on professional antigen-presenting cells such as
dendritic cells,
macrophages, and
B cells, these molecules present peptides derived from extracellular proteins to
CD4+ T cells.
Structure of MHC Molecules
Both Class I and Class II MHC molecules have a similar structural framework:- Class I MHC Molecules: Composed of a heavy chain (α-chain) and a β2-microglobulin. The α-chain has three domains (α1, α2, α3), with the α1 and α2 domains forming the peptide-binding groove.
- Class II MHC Molecules: Consist of two chains (α and β), each with two domains (α1, α2 and β1, β2). The α1 and β1 domains form the peptide-binding cleft.
Function of MHC Molecules
The primary function of MHC molecules is to present antigenic peptides to T cells. This is crucial for the activation of the adaptive immune response. Upon recognizing an antigen-MHC complex, T cells can initiate an immune response to eliminate the pathogen or infected cells.Genetic Organization of MHC
The genes encoding MHC molecules are located on chromosome 6 in humans. This region is highly variable and has several loci, including HLA-A, HLA-B, and HLA-C for Class I MHC, and HLA-DP, HLA-DQ, and HLA-DR for Class II MHC. The high degree of polymorphism in these genes ensures a broad range of antigen presentation.Clinical Relevance of MHC
The MHC plays a pivotal role in
transplantation. Mismatched MHC molecules between donor and recipient can lead to graft rejection. Thus, HLA typing is essential for organ and bone marrow transplants. MHC is also involved in the susceptibility to various autoimmune diseases, such as
rheumatoid arthritis and
type 1 diabetes.
Histological Identification of MHC Molecules
In histology, MHC molecules can be identified using immunohistochemistry techniques. Specific antibodies against MHC class I or II molecules are used to stain tissue sections, allowing for the visualization of MHC expression patterns in different cell types and tissues.Conclusion
The Major Histocompatibility Complex is critical for immune system function, playing a significant role in antigen presentation and immune response activation. Understanding the structure, function, and genetic organization of MHC molecules is essential in fields such as immunology, histology, and clinical medicine, particularly in the context of transplantation and autoimmune diseases.
