Fig. 12: Cross-Presentation of Exogenous Antigens to MHC-I Molecules
by a Dendritic Cell:
Transfer of Peptides from Phagolysosome to Vesicles Containing MHC-I Molecules

Exogenous antigens can be cross-presented to MHC-I molecules by certain dendritic cells. One proposed mechanism is via the direct transfer of peptides from the phagolysosome into the vesicles containing the MHC-I molecules in the dendritic cell.

1. An exogenous antigen is engulfed and placed in a phagosome.
2. After a lysosome fuses with the phagosome forming a phagolysosome.

3. Proteins from the exogenous antigen are degraded by lysosomal proteases into peptides.
4. The peptides are directly transfered from the phagolysosome to the vesicles containing the MHC-I molecules.
5 . The MHC-I molecules with bound peptides are then transported to the surface of the dendritic cell where they can be presented to naive T8-lymphocytes.

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