How the Immune System Recognizes the Myriad of Antigens: Diversity of MHC (HLA) and TCR Molecules

The immune system must distinguish a vast diversity of pathogens while avoiding harmful responses to self. This specificity is achieved through two unique families of molecules: T-cell receptors (TCRs) on T lymphocytes and major histocompatibility complex (MHC) proteins, known as the human leukocyte antigen molecules in humans, on antigen-presenting cells. MHC molecules provide the structural platform for displaying short peptide fragments derived from intracellular or extracellular proteins. MHC class I molecules present endogenous peptides to CD8⁺ cytotoxic T cells, while MHC class II molecules present exogenous peptides to CD4⁺ helper T cells. The extraordinary diversity of MHC genes, generated through polymorphism and codominant expression, enables a single individual to present an enormous range of peptides, thereby enhancing immune surveillance. However, it also creates barriers to organ transplantation. TCRs, by contrast, achieve their diversity through somatic gene rearrangements. Random recombination of variable (V), diversity (D), and joining (J) gene segments, along with junctional modifications mediated by RAG proteins and terminal deoxynucleotidyl transferase, produces a nearly limitless TCR repertoire. During thymic development, positive selection ensures recognition of self-MHC, while negative selection eliminates autoreactive clones, establishing central tolerance. Together, the polymorphism of MHC molecules and the genetic recombination of TCR genes equip the immune system with the capacity to recognize countless foreign antigens while maintaining self-tolerance. This dual system exemplifies the elegant molecular coevolution of antigen presentation and recognition, forming the foundation of adaptive immunity.