Antigen presentation

Antigen presentation is the cellular process by which protein fragments — peptides — are loaded onto MHC molecules and displayed on the cell surface for recognition by T cells. It is the translation layer between intracellular biochemistry and immune surveillance: the cell's proteome is continuously sampled, fragmented, and screened, and the resulting peptides are broadcast as a surface-readable status report. Without antigen presentation, the adaptive immune system would be blind to intracellular events, and cytotoxic T cells would have no targets to kill.

The process differs fundamentally between MHC class I and MHC class II pathways. Class I presentation samples proteins synthesized in the cytosol — typically viral proteins produced in infected cells or mutated proteins in tumors — and loads them onto MHC class I in the endoplasmic reticulum. Class II presentation samples extracellular proteins internalized by phagocytosis or endocytosis, degrades them in specialized compartments, and loads peptides onto MHC class II for display to helper T cells. These parallel pathways ensure that both intracellular and extracellular threats can be communicated to the appropriate T cell subset.