How do T cells recognize antigens?

T cells recognize antigens through a molecular handshake that depends on presentation, specificity, and cellular context. Antigens are not usually seen by T cells as whole pathogens but as short peptide fragments displayed on cell surfaces by major histocompatibility complex molecules. Peter Doherty of the University of Melbourne and Rolf Zinkernagel of the University of Zurich established the principle that T cells recognize antigen only when it is bound to MHC molecules, a concept known as MHC restriction. The structural basis for this interaction was clarified by Pamela Bjorkman at the California Institute of Technology and Don Wiley at Harvard University, who showed how peptide and MHC together form the surface that the T cell receptor inspects.

Molecular recognition

The T cell receptor is a membrane protein formed by alpha and beta chains whose genes were identified by Tak Wah Mak at the University of Toronto. The receptor engages a peptide lodged in the binding groove of MHC class I or class II. MHC class I molecules present peptides derived from proteins made inside the cell to CD8 positive T cells, which often become cytotoxic and kill infected or malignant cells. MHC class II molecules present peptides from extracellular proteins that have been internalized and processed by antigen presenting cells to CD4 positive helper T cells, which coordinate broader immune responses. Co-receptors CD4 and CD8 stabilize the interaction and recruit intracellular kinases that translate receptor binding into signaling cascades, leading to clonal expansion and differentiation when the signal exceeds activation thresholds.

Antigen processing and presentation are controlled by specialized cells. Ralph Steinman of Rockefeller University discovered dendritic cells and demonstrated their central role in capturing antigens, migrating to lymph nodes, and activating naive T cells. Intracellular proteases, the proteasome, and peptide transporters determine which fragments reach MHC molecules, influencing which epitopes are visible to T cells and shaping the specificity of the immune response.

Biological and societal consequences

The way T cells detect antigens has direct relevance for infection control, autoimmunity, and cancer therapy. Immune checkpoint biology translated into clinical benefit when James Allison at the University of Texas MD Anderson Cancer Center and Tasuku Honjo at Kyoto University developed strategies to release inhibitory brakes on T cells, leading to durable cancer remissions in some patients. Conversely, errors in antigen recognition or tolerance can cause autoimmune disease when self peptides presented by particular human leukocyte antigen variants are mistaken for foreign. Human leukocyte antigen diversity varies widely across populations and regions, a fact that influences susceptibility to certain infections, vaccine responses, and transplant compatibility. Environmental pressures from endemic pathogens have shaped HLA distribution in different territories, with cultural and socioeconomic factors further affecting access to diagnostics and treatments that depend on understanding T cell recognition. Understanding the precise molecular interactions between T cell receptors, peptides, and MHC remains essential for rational vaccine design, transplant matching, and targeted immunotherapies that aim to harness T cells without triggering harmful autoimmunity.