Major histocompatibility complex
Major histocompatibility complex (MHC) molecules are cell-surface proteins that display peptide fragments — derived from proteins inside the cell — to the immune system's surveillance apparatus. They are the central information platform of adaptive immunity: without MHC, T cells cannot see what is happening inside other cells, and the distinction between healthy tissue and infected or transformed tissue collapses into guesswork. Every nucleated cell in the body expresses MHC class I; professional antigen-presenting cells express MHC class II as well. The peptides they display are not random: they are the cell's ongoing report on its internal protein content, and T cells read this report continuously.
Structure and the Two Classes
MHC molecules are built for a single purpose: bind a peptide and present it to a T cell receptor. An MHC protein has a deep groove — the peptide-binding cleft — that accommodates short peptide fragments, typically 8-10 amino acids for MHC class I and 13-18 for MHC class II. The floor and walls of this cleft are the most polymorphic regions of the human genome: in a population, there are thousands of variants, each with slightly different binding preferences. This polymorphism is not decorative. It is a population-level bet-hedging strategy: if a pathogen evolves to evade one MHC variant, other variants in the population can still present its peptides.
MHC class I molecules are expressed on all nucleated cells. They present peptides derived from proteins synthesized inside the cell — including viral proteins produced during infection and mutated proteins in cancer cells. A cytotoxic T cell that recognizes a foreign or abnormal peptide on MHC class I will kill the presenting cell. This is the mechanism of cell-mediated immunity: the infected cell announces its own pathology, and the immune system responds with lethal precision.
MHC class II molecules are expressed primarily on professional antigen-presenting cells — dendritic cells, macrophages, and B cells. They present peptides derived from extracellular proteins that the cell has internalized and processed. A helper T cell that recognizes its cognate peptide on MHC class II becomes activated and coordinates the broader immune response. The class I/class II division is a functional specialization: class I surveys internal threats, class II surveys external threats that have been sampled and reported.
MHC Restriction and the Logic of Self
The concept of MHC restriction is one of the most important discoveries in immunology. A T cell selected in one individual's thymus recognizes antigen only when presented by that individual's own MHC molecules. The T cell receptor is calibrated to self-MHC during positive selection; its antigen recognition is inseparable from its MHC recognition. This means that T cell immunity is fundamentally selfish: a T cell cannot protect a cell that does not display the right MHC.
This selfishness has profound consequences. Organ transplantation fails because the recipient's T cells reject donor tissue displaying foreign MHC — a phenomenon called alloreactivity. The immune system treats the transplanted organ as infected, not because it is, but because its MHC molecules are foreign. MHC matching between donor and recipient is therefore the primary determinant of transplant success. The more closely matched the MHC, the less violent the rejection.
MHC restriction also shapes the co-evolutionary arms race between hosts and pathogens. Pathogens evolve mechanisms to interfere with MHC expression — reducing peptide presentation, blocking transport, or diverting MHC molecules from the cell surface. Viruses like CMV and HIV encode proteins specifically designed to downregulate MHC class I, rendering infected cells invisible to cytotoxic T cells. The host responds with natural killer cells that kill cells with abnormally low MHC expression — a backup surveillance system that detects the attempt to hide.
MHC in Disease and Population Genetics
The extreme polymorphism of MHC genes makes them powerful markers in population genetics and disease association. Certain MHC alleles are strongly associated with autoimmune diseases: HLA-B27 with ankylosing spondylitis, HLA-DR3 and DR4 with type 1 diabetes, HLA-DQ2 with celiac disease. These associations are among the strongest in all of genetics, with odds ratios that dwarf most complex disease associations. The same MHC variants that protect against one pathogen may predispose to self-attack — a tradeoff encoded in the same molecular structure.
In infectious disease, MHC diversity determines susceptibility. Individuals with MHC variants that can present peptides from a particular pathogen mount stronger T cell responses and clear infection faster. Populations with greater MHC diversity are more resilient to emerging pathogens. This is why MHC polymorphism is maintained by balancing selection — heterozygotes have an advantage because they can present a broader range of peptides. The immune system's genetic diversity is not an accident; it is an evolved response to the unpredictability of pathogens.
The peptide-MHC complex is not merely a molecular structure. It is an information interface — the point at which the internal state of a cell becomes legible to the immune network. Understanding MHC is not about memorizing HLA types. It is about recognizing that the immune system solves the problem of internal surveillance through a molecular display system that converts protein content into a surface-readable code. The cell becomes its own informant.
The MHC is not a passive scaffold. It is an active participant in immune recognition, shaping the peptide repertoire through binding preferences, editing the display through intracellular trafficking, and determining which T cells live and die during thymic selection. To treat MHC as merely a 'presentation molecule' is to mistake the stage for the play. The MHC defines what the immune system can see — and what it cannot see is as consequential as what it can. The MHC is the immune system's epistemology, and like all epistemologies, it has blind spots that shape the structure of knowledge.