MHC class I (major histocompatibility complex class I) is a protein found on the surface of nearly every cell in your body. Its job is to display small fragments of whatever proteins the cell is making, essentially giving your immune system a window into what’s happening inside each cell. If a cell is healthy, MHC I shows normal protein fragments and immune cells move on. If the cell is infected by a virus or has become cancerous, MHC I displays abnormal fragments that trigger an immune attack.
How MHC I Works
Every cell constantly breaks down and recycles its own proteins. MHC I takes advantage of this process. Inside the cell, a structure called the proteasome chops proteins into small peptide fragments, typically 8 to 11 amino acids long (most commonly 9). These fragments are shuttled into a compartment called the endoplasmic reticulum, where they’re loaded onto waiting MHC I molecules. Once a fragment locks into place, the complete package travels to the cell surface and sits there like a tiny display shelf.
This system means your cells are always broadcasting a sample of their internal contents. A healthy cell displays fragments of normal human proteins. A virus-infected cell, though, is churning out viral proteins, and pieces of those viral proteins end up on MHC I too. The same goes for cancer cells producing mutant proteins. That abnormal display is what gets a cell flagged for destruction.
The Immune Cells That Read MHC I
The primary readers of MHC I are CD8+ T cells, also called cytotoxic (cell-killing) T cells. These cells patrol the body and physically dock with MHC I molecules on cell surfaces. The CD8 protein on the T cell acts like a co-receptor, helping it grip the MHC I molecule while the T cell’s main receptor inspects the peptide fragment on display. If the fragment looks abnormal, the T cell kills that cell directly.
Natural killer (NK) cells interact with MHC I differently. Instead of checking what’s being displayed, NK cells check whether MHC I is present at all. One end of the MHC I molecule serves as a binding site for an inhibitory receptor on NK cells. When MHC I is present, it sends a “don’t kill me” signal. When MHC I is missing or reduced, NK cells interpret that as a sign something is wrong and attack. This creates a two-layered defense: CD8+ T cells catch cells displaying abnormal contents, and NK cells catch cells that have tried to hide their contents entirely.
Where MHC I Appears in the Body
MHC I is expressed on the surface of all nucleated cells, which covers the vast majority of cells in your body. Red blood cells are one notable exception. Because mature red blood cells lack a nucleus (and the DNA needed to produce MHC I), they don’t display these molecules. This is one reason blood type matching for transfusions relies on a different set of surface markers rather than MHC matching.
The Genes Behind MHC I
In humans, MHC I molecules are encoded by genes in the HLA (human leukocyte antigen) system, located in a 3.7 million base-pair stretch on chromosome 6. The three main genes are HLA-A, HLA-B, and HLA-C. These genes are among the most variable in the entire human genome, which means the MHC I molecules on your cells are slightly different from those on almost everyone else’s cells. You inherit one set from each parent, giving you up to six different versions of classical MHC I.
This extreme genetic diversity exists because it benefits populations as a whole. Different MHC I variants are better at displaying fragments from different pathogens. A population with many different HLA types is harder for any single virus to evade completely.
How MHC I Differs From MHC II
MHC I and MHC II serve related but distinct purposes. MHC I appears on nearly all nucleated cells and shows fragments from proteins made inside the cell. It communicates with CD8+ killer T cells. MHC II, by contrast, appears only on specialized immune cells (like dendritic cells and macrophages) and shows fragments from things the cell has swallowed from outside, such as bacteria or debris. MHC II communicates with CD4+ helper T cells, which coordinate the broader immune response rather than killing cells directly.
Structurally, MHC I has a peptide-binding groove that is closed at both ends, restricting it to short peptides of 8 to 11 amino acids. MHC II has an open-ended groove that accommodates longer fragments, typically 12 to 25 amino acids, which extend beyond both sides. Think of MHC I as a snug pocket and MHC II as an open channel.
How Viruses and Cancer Exploit MHC I
Because MHC I is the primary way infected or abnormal cells get identified, viruses have evolved numerous strategies to shut it down. Herpesviruses produce proteins that degrade MHC I molecules and block the transport of peptide fragments. HIV-1 uses a protein called Nef to reroute newly made MHC I molecules away from the cell surface and into compartments where they’re destroyed. Human cytomegalovirus encodes proteins that hijack the cell’s own quality-control machinery to break down MHC I. Even SARS-CoV-2 has a protein (ORF6) that suppresses the pathway responsible for activating MHC I production. Influenza viruses cause a noticeable reduction in surface MHC I during the later stages of infection.
Cancer cells use similar tricks. Tumors frequently lose or reduce MHC I expression, which helps them avoid detection by CD8+ T cells. Research has shown that a key regulator of MHC I production (called NLRC5) is a common target for immune evasion in cancer. This is one reason why some immunotherapy approaches focus on restoring or bypassing MHC I function in tumors. When cancer cells lose MHC I entirely, they become invisible to killer T cells but may become more vulnerable to NK cells, which attack cells lacking MHC I.
MHC I in Transplant Medicine
Because MHC I molecules vary so much between individuals, they’re a major factor in organ transplant rejection. Your immune system recognizes another person’s MHC I as foreign, just as it would recognize a virus-infected cell. This is why HLA matching between donor and recipient is standard practice for kidney, heart, and bone marrow transplants. Better matching has been proven to increase graft survival and reduce the incidence of acute and chronic rejection. In kidney transplantation, the benefit of close HLA matching is well established. Even in liver transplantation, where the liver was historically considered more immune-tolerant, meta-analyses have found that fewer HLA mismatches (0 to 2 versus 3 to 6) significantly reduce acute rejection rates.
MHC I and Autoimmune Disease
Certain HLA variants are strongly linked to autoimmune conditions. The most studied example is HLA-B27, a specific version of MHC I encoded by the HLA-B gene. HLA-B27 is present in up to 90% of patients with ankylosing spondylitis, an inflammatory disease affecting the spine and large joints. This single gene variant accounts for roughly 20% of the disease’s heritability. The association between HLA-B27 and ankylosing spondylitis, first discovered in 1973, remains one of the strongest known links between a single gene and an autoimmune disease.
Not all HLA-B27 subtypes carry the same risk. The subtypes most associated with ankylosing spondylitis vary by population: HLA-B*27:05 in Caucasians, HLA-B*27:04 in Chinese populations, and HLA-B*27:02 in Mediterranean populations. Interestingly, two subtypes, HLA-B*27:06 and HLA-B*27:09, show no association with the disease at all. Carrying HLA-B27 doesn’t guarantee you’ll develop ankylosing spondylitis, and a proportion of cases occur without HLA-B27 entirely, but it remains one of the most important genetic risk factors.