The known unknowns of antigen processing and presentation

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Abstract
Antigen processing and presentation is the mechanism by which whole antigens are degraded and loaded onto MHC molecules for display on the cell surface for recognition by T cells. Both macrophages and dendritic cells (DCs) are considered professional antigen-presenting cells, although DCs possess the unique capacity to activate naive T cells. DCs phagocytose antigens and whole microorganisms and place them into membrane-delimited compartments termed phagosomes. These structures are modified over time and ultimately fuse with lysosomes to form phagolysosomes. Cross-presentation allows DCs to take up antigens from the extracellular environment and present them on MHC class I molecules for CD8+ T-cell activation. Although the exact trafficking patterns of antigens are not known, many hypotheses have been generated, including the recruitment of the endoplasmic reticulum dislocation machinery to the phagosome and protein-independent passing of antigens from the phagosome or phagolysosome into the cytosol. The autophagy machinery is thought to have an important role in the generation of peptide antigens for MHC class II molecules.