Major histocompatibility complex

Major histocompatibility complex. The major histocompatibility complex is a large genomic region or gene family found in most vertebrates that encodes proteins critical for the adaptive immune system. Its most well-known function is to present peptide antigens to T cells, initiating immune responses. The genes within this complex are the most polymorphic in the Human genome, playing a central role in transplant rejection and susceptibility to a wide array of diseases. Research on this complex, pioneered by scientists like Baruj Benacerraf, Jean Dausset, and George Snell (who coined the term), was foundational to modern Immunology and earned them the Nobel Prize in Physiology or Medicine in 1980.

Structure and classification

The proteins encoded by the major histocompatibility complex are divided into two primary classes based on structure, function, and tissue distribution. MHC class I molecules are expressed on the surface of nearly all nucleated cells and present peptides derived from intracellular proteins, such as those from viruses or cancerous transformations. These molecules are heterodimers consisting of a heavy α chain and a lighter chain called Beta-2 microglobulin. In contrast, MHC class II molecules are typically expressed on professional antigen-presenting cells like dendritic cells, macrophages, and B cells. They present peptides from extracellular pathogens that have been engulfed and processed. A third class, MHC class III, encodes various other immune proteins, including components of the Complement system and cytokines like Tumor necrosis factor.

Function in the immune system

The primary function of these molecules is antigen presentation, a process essential for the activation of the adaptive immune response. MHC class I molecules present endogenous peptides to CD8+ cytotoxic T cells, which can then identify and eliminate infected or malignant cells. MHC class II molecules present exogenous peptides to CD4+ helper T cells, which orchestrate the broader immune response by activating other cells like B cells and macrophages. This discrimination between self and non-self peptides is crucial for immune surveillance; failures in this process can lead to autoimmune disorders. The interaction between the presented peptide and the T-cell receptor is a central event in immunology, studied extensively at institutions like the National Institutes of Health and the La Jolla Institute for Immunology.

Genetics and polymorphism

The genes of the major histocompatibility complex are highly polymorphic, meaning they exist in many different allelic forms within a population. In humans, this complex is located on the short arm of Chromosome 6 and is known as the Human leukocyte antigen system. This extreme polymorphism is thought to be driven by selective pressure from pathogens, as a diverse array of alleles increases the likelihood that at least some individuals in a population can present antigens from any given pathogen. The genes are also characterized by Linkage disequilibrium, where certain sets of alleles are inherited together more frequently than expected. This genetic region has been a major focus of studies in Population genetics and was pivotal in the work of researchers like Peter Medawar on Immunological tolerance.

Role in disease and medicine

Variation in the major histocompatibility complex is a significant factor in disease susceptibility, transplant compatibility, and Pharmacogenomics. Specific alleles are strongly associated with autoimmune diseases such as Type 1 diabetes, Rheumatoid arthritis, Multiple sclerosis, and Celiac disease. In transplantation, matching HLA types between donor and recipient is critical to minimize the risk of Graft-versus-host disease and transplant rejection, a principle established through early work on inbred mouse strains. Furthermore, certain alleles can influence drug responses, such as the association between HLA-B*57:01 and hypersensitivity to the HIV drug Abacavir, leading to pre-treatment screening protocols.

Evolution

The major histocompatibility complex is believed to have originated in the earliest jawed vertebrates, with homologous regions identified in species from Cartilaginous fish to mammals. The driving force behind its evolution is the constant arms race with rapidly evolving pathogens, leading to the maintenance of high polymorphism through mechanisms like Balancing selection and Heterozygote advantage. Comparative genomics of species like the Zebrafish, Chicken, and the Tasmanian devil has revealed conservation of core functions but also species-specific expansions and adaptations. The study of its evolution provides insights into Vertebrate immune history and the selective pressures shaped by epidemics throughout time, contributing to fields like Paleogenomics.

Category:Immunology Category:Genetics Category:Proteins