The MHC System: How Your Cells Display Antigens for Immune Recognition

What the MHC System Does

T cells cannot recognize free-floating antigens the way antibodies can. Instead, they require antigens to be processed into short peptide fragments and displayed on MHC molecules on the surface of cells. This requirement, called MHC restriction, was discovered by Peter Doherty and Rolf Zinkernagel in 1974, earning them the Nobel Prize in Physiology or Medicine in 1996. Their experiments showed that cytotoxic T cells could only kill virus-infected cells if those cells shared the same MHC molecules as the T cells themselves. If the MHC molecules were different, the T cells ignored the infected cells completely, even though the viral antigens were identical.

The biological logic behind MHC restriction is that the MHC molecule and the peptide it carries form a combined structure that the T cell receptor recognizes as a single unit. The TCR contacts both the MHC molecule and the peptide simultaneously, with roughly half of the TCR's binding energy directed toward the MHC molecule and half toward the peptide. This dual recognition ensures that T cells can only respond to antigens that have been properly processed and presented by the body's own cells, preventing them from reacting to free antigens in the blood or tissue fluids where context about the antigen's source would be lost.

The MHC region occupies approximately 4 megabases on the short arm of chromosome 6 (6p21.3) in humans. It contains over 200 genes, making it one of the most gene-dense regions in the human genome. The classical MHC genes encoding the antigen-presenting molecules are divided into class I (HLA-A, HLA-B, HLA-C) and class II (HLA-DR, HLA-DQ, HLA-DP). The region also contains class III genes encoding complement proteins (C2, C4, factor B), TNF-alpha, and several heat shock proteins, though these do not directly participate in antigen presentation.

MHC Class I: Showing What Is Inside Every Cell

MHC class I molecules are expressed on the surface of virtually every nucleated cell in the human body. Their function is to continuously sample the proteins being produced inside the cell and display representative peptide fragments on the cell surface for inspection by CD8+ cytotoxic T cells. This system serves as a molecular window that allows T cells to monitor the internal health of cells without entering them.

The class I antigen presentation pathway begins in the cytoplasm, where proteins are degraded by the proteasome, a large barrel-shaped protease complex. The proteasome chops proteins into peptide fragments, typically 8 to 11 amino acids long. These peptides are transported from the cytoplasm into the endoplasmic reticulum (ER) by the transporter associated with antigen processing (TAP), a heterodimeric ABC transporter composed of TAP1 and TAP2 subunits. Inside the ER, peptides are loaded onto newly synthesized MHC class I molecules with the help of the peptide-loading complex, which includes the chaperone proteins tapasin, calreticulin, and ERp57.

Only peptides that bind with sufficient affinity to the peptide-binding groove of the MHC class I molecule are stably loaded. The binding groove accommodates peptides of a specific length and with anchor residues at defined positions that match the particular HLA allele. For example, HLA-A*02:01, one of the most common class I alleles in people of European descent, preferentially binds peptides with leucine or methionine at position 2 and valine or leucine at the C-terminal position. Each HLA allele has a different binding preference, which means different individuals present different peptides from the same protein, a fact with major implications for immune responses to infections and vaccines.

Once loaded with a peptide, the stable MHC class I-peptide complex is transported to the cell surface, where it remains for several hours before being internalized and replaced. In a healthy cell, all displayed peptides derive from normal cellular proteins, and circulating cytotoxic T cells recognize these as "self" and move on without reacting. In a virus-infected cell, viral proteins are processed through the same pathway, and viral peptide-MHC class I complexes appear on the surface alongside self-peptides. A cytotoxic T cell whose TCR recognizes a viral peptide-MHC complex will activate and kill the infected cell.

Many viruses have evolved mechanisms to evade MHC class I presentation. Herpes simplex virus produces a protein (ICP47) that blocks TAP, preventing peptide transport into the ER. Cytomegalovirus produces multiple proteins that interfere with class I assembly and transport. Adenovirus downregulates class I surface expression. HIV's Nef protein accelerates the internalization of MHC class I molecules from the cell surface. These evasion strategies underscore how critical the class I pathway is for antiviral immunity and why viruses invest significant genetic resources in circumventing it.

MHC Class II: Presenting External Threats

MHC class II molecules have a more restricted expression pattern than class I. They are constitutively expressed only on professional antigen-presenting cells: dendritic cells, macrophages, B cells, and thymic epithelial cells. Other cell types, including endothelial cells and epithelial cells, can be induced to express class II molecules by IFN-gamma during inflammation. Class II molecules present peptides derived from extracellular proteins to CD4+ helper T cells, providing the signal that activates the adaptive immune system's coordinating arm.

The class II antigen presentation pathway begins when an antigen-presenting cell engulfs extracellular material through phagocytosis, receptor-mediated endocytosis, or macropinocytosis. The engulfed material enters the endosomal-lysosomal pathway, where progressively acidic pH and proteolytic enzymes (cathepsins) degrade the proteins into peptide fragments, typically 13 to 25 amino acids long, longer than class I peptides because the class II binding groove is open at both ends rather than closed.

MHC class II molecules are synthesized in the ER, where they associate with a protein called the invariant chain (Ii, CD74). The invariant chain serves two purposes: it blocks the peptide-binding groove, preventing premature binding of ER-resident peptides (which would otherwise be presented as if they were foreign), and it directs the class II molecule into the endosomal pathway through a targeting signal in its cytoplasmic tail. In the late endosomal compartment, the invariant chain is progressively degraded by cathepsins, leaving a small fragment called CLIP (class II-associated invariant chain peptide) occupying the binding groove. The molecule HLA-DM then catalyzes the exchange of CLIP for higher-affinity peptides derived from the engulfed material. The stable class II-peptide complex is transported to the cell surface for presentation to CD4+ T cells.

This separation of the class I and class II pathways ensures that cytotoxic T cells receive information about what is happening inside cells (class I) while helper T cells receive information about what is happening in the extracellular environment (class II). The segregation is not absolute, however. A process called cross-presentation allows certain dendritic cells to load extracellular antigens onto MHC class I molecules, enabling the activation of cytotoxic T cells against pathogens that do not directly infect dendritic cells. Cross-presentation is critical for initiating CD8+ T cell responses against tumors and viruses that preferentially infect non-immune cells.

HLA Polymorphism: The Most Variable Genes in the Human Genome

The HLA genes are the most polymorphic coding genes known in any vertebrate species. As of 2026, the IPD-IMGT/HLA Database lists over 36,000 distinct HLA alleles across all classical loci. HLA-B alone has over 8,000 known alleles. This extraordinary diversity means that the probability of two unrelated individuals having identical HLA types at all classical loci is extremely low, roughly 1 in several million in most populations.

Most HLA polymorphism is concentrated in the regions of the gene encoding the peptide-binding groove, which directly determines which peptides each allele can present. Different alleles bind different sets of peptides, so individuals with different HLA types present different fragments of the same pathogen. At the population level, this diversity ensures that no single pathogen can evolve to evade presentation by all HLA alleles simultaneously. Even if a virus mutates to escape presentation by one common HLA allele, individuals carrying other alleles will still present and respond to the virus effectively.

The selective advantage of HLA diversity has been demonstrated both theoretically and empirically. HLA heterozygotes, individuals who inherit different alleles from each parent, present a wider range of peptides than homozygotes and generally mount more effective immune responses against diverse pathogens. Studies of HIV progression have shown that HLA heterozygosity is associated with slower disease progression, presumably because a broader peptide presentation repertoire allows the immune system to target more viral epitopes. Certain specific HLA alleles, notably HLA-B*57:01 and HLA-B*27:05, are strongly associated with better control of HIV infection because they present immunodominant viral peptides that elicit particularly effective cytotoxic T cell responses.

The mechanisms maintaining HLA diversity include balancing selection (heterozygote advantage), frequency-dependent selection (rare alleles confer advantage because pathogens are less likely to have evolved evasion strategies for uncommon alleles), and selection driven by pathogen diversity in different geographic regions. These forces have maintained HLA polymorphism for millions of years, with some allele lineages predating the divergence of humans from other great apes.

HLA and Disease Associations

HLA type is the strongest genetic risk factor for many autoimmune and inflammatory diseases. The associations are remarkably specific: HLA-B*27 confers a relative risk of approximately 100 for ankylosing spondylitis (a chronic inflammatory arthritis of the spine), making it one of the strongest gene-disease associations in all of human genetics. HLA-DQ2 and HLA-DQ8 are present in virtually 100 percent of patients with celiac disease, compared to roughly 30 percent of the general population. HLA-DR4 is associated with rheumatoid arthritis, HLA-DR3 with type 1 diabetes and systemic lupus erythematosus, and HLA-DQ6 with narcolepsy.

The prevailing explanation for most HLA-disease associations is that specific HLA alleles present self-peptides in a way that activates autoreactive T cells. In celiac disease, HLA-DQ2 and HLA-DQ8 present deamidated gluten peptides to CD4+ T cells in the small intestinal mucosa, triggering an inflammatory response that damages the intestinal villi. Without these specific HLA molecules, the gluten peptides cannot be presented, and celiac disease does not develop. This explains why HLA-DQ2/DQ8 is necessary but not sufficient for celiac disease: gluten exposure, and additional genetic and environmental factors, are also required.

HLA alleles also influence drug hypersensitivity reactions. HLA-B*57:01 is strongly associated with abacavir hypersensitivity, a potentially fatal reaction to the HIV drug abacavir. The HLA molecule presents abacavir (or abacavir-modified self-peptides) to T cells, triggering an immune response against the patient's own tissues. Pharmacogenomic testing for HLA-B*57:01 before prescribing abacavir has virtually eliminated this adverse reaction, making it one of the most successful examples of personalized medicine. Similar HLA-linked drug hypersensitivities have been identified for carbamazepine (HLA-B*15:02 in Southeast Asian populations) and allopurinol (HLA-B*58:01).

MHC in Transplantation

The MHC system was originally named for its role in determining whether tissue grafts between individuals would be accepted or rejected. Transplant rejection occurs because the recipient's T cells recognize donor MHC molecules as foreign. This can happen through two distinct pathways: direct recognition, in which recipient T cells bind directly to intact donor MHC molecules on transplanted cells (treating the foreign MHC as if it were a self-MHC carrying a foreign peptide), and indirect recognition, in which recipient antigen-presenting cells process shed donor MHC molecules and present peptide fragments of donor MHC on recipient class II molecules.

HLA matching between donor and recipient reduces, but does not eliminate, the risk of transplant rejection. For kidney transplants, matching at HLA-A, HLA-B, and HLA-DR improves graft survival. For bone marrow transplants, which also carry the risk of graft-versus-host disease (in which donor T cells attack recipient tissues), high-resolution matching at HLA-A, HLA-B, HLA-C, HLA-DRB1, HLA-DQB1, and often HLA-DPB1 is required. Even with perfect 10/10 or 12/12 HLA matching, minor histocompatibility antigens (polymorphic peptides from non-HLA genes presented on shared MHC molecules) can trigger rejection or graft-versus-host disease, which is why immunosuppressive drugs remain necessary for virtually all transplant recipients.

The extreme polymorphism of HLA genes makes finding matched unrelated donors challenging. Bone marrow registries such as the National Marrow Donor Program (Be The Match) in the United States maintain databases of millions of volunteer donors typed at high-resolution HLA loci. Despite these large registries, patients from underrepresented ethnic groups often struggle to find matched donors because HLA allele frequencies vary significantly between populations. Haploidentical transplantation, using a half-matched family member as the donor, has become increasingly feasible with improved immunosuppressive protocols, expanding access to transplantation for patients without matched donors.