Antigen Processing & Presentation

MHC-mediated display of peptide antigens to T cells.

Overview

MHC class I presents intracellular peptides (from proteasomal degradation) to CD8+ T cells. Peptides are transported to the ER by TAP, loaded onto MHC I with the help of tapasin, and displayed at the cell surface. MHC class II presents extracellular antigens (from endosomal degradation) to CD4+ T cells. Invariant chain (Ii/CD74) blocks premature peptide loading; HLA-DM catalyzes peptide exchange.

Cellular Location

ER, endosomes, cell surface

Clinical Significance

Critical for adaptive immunity; HLA polymorphisms determine disease susceptibility; defects cause immunodeficiency; cross-presentation is key for antitumor immunity.

Key Molecules

MHC class I MHC class II β2-microglobulin Peptides TAP1/TAP2 Tapasin Invariant chain (Ii)HLA-DM CLIP

Key Enzymes

Proteasome (immunoproteasome)Cathepsins (B, D, L, S)ERAP1/2 (aminopeptidases)TAP (ABC transporter)

Related Pathways

T Cell Receptor Signaling

Antigen-specific activation of T lymphocytes via TCR-MHC interaction.

Toll-Like Receptor (TLR) Signaling

Pattern recognition receptors activating innate immune responses.