Antigen Presentation

Abstract

Tlymphocytes recognize antigens only in the context of self-MHC molecules on the surface of accessory cells. During processing, intact protein antigens are degraded into peptide fragments. Most epitopes that T cells recognize are peptide chain fragments. B cells and T cells often recognize different epitopes of an antigen leading to both antibody and cell-mediated immune responses. Before antigen can bind to MHC molecules, it must be processed into peptides in the intracellular organelles. CD4⁺ helper T lymphocytes recognize antigens in the context of class II MHC molecules, a process known as class II MHC-restriction. By contrast, CD8⁺ cytotoxic T lymphocytes recognize antigens in the context of class I molecules, which is known as class I MHCrestriction. Following the generation of peptides by proteolytic degradation in antigen-presenting cells, peptideMHC complexes are presented on the surface of antigen-presenting cells where they may be recognized by T lymphocytes. Antigens derived from either intracellular or extra-cellular proteins may be processed to produce peptides from either self or foreign proteins that are presented by surface MHC molecules to T cells. In the class II MHC processing pathway, professional antigen-presenting cells, such as macrophages, dendritic cells or B lymphocytes incorporate extra-cellular proteins into endosomes where they are processed (Figures 1 and 2). Enzymes within the vesicles of the endosomal pathway cleave proteins in the acidic environment.