antigenpeptides

Antigen peptides are short sequences of amino acids derived from larger proteins that can be recognized by the adaptive immune system when displayed by major histocompatibility complex (MHC) molecules on antigen-presenting cells. These peptide fragments, or epitopes, are central to how T cells detect and respond to pathogens, vaccines, or altered self-proteins in autoimmunity.

Peptide generation and presentation occur mainly through two pathways. In the endogenous pathway, cytosolic proteins are degraded by the proteasome; resulting peptides are transported into the endoplasmic reticulum by the TAP transporter, where they are loaded onto MHC class I molecules and presented on the cell surface to CD8+ T cells. In the exogenous pathway, extracellular proteins are internalized and processed in endosomes/lysosomes; peptides are loaded onto MHC class II molecules and presented to CD4+ T helper cells. The peptide–MHC complex is then surveyed by T-cell receptors, which can trigger an immune response if the peptide is foreign or pathogenic.

Peptides that bind MHC I are typically 8–11 amino acids long, whereas MHC II–bound peptides are longer, usually 13–25 residues. Antigen peptides can be linear (continuous) sequences or, for antibodies, part of conformational epitopes dependent on protein folding.

Applications include peptide vaccines, diagnostic assays, and epitope mapping. Synthetic antigenic peptides and computationally predicted epitopes are widely used to study immune responses and to design targeted immunotherapies.