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Animated version (Shockwave Flash)
Stephen Man
Figure 4. Degradation and transport of antigens that bind major histocompatibility complex (MHC) class II molecules. (a) In an antigen-presenting cell (APC), newly synthesised MHC class II molecules bind the invariant chain (IC), which prevents binding of peptides that are present in the endoplasmic reticulum (ER). (b) The IC allows transport of MHC class II molecules from the ER into the Golgi apparatus to acidified endosomes. (c) Endosomes contain peptides that are derived from either resident pathogens (e.g. bacteria) or (d) engulfed extracellular proteins (or pathogens) (e) in the phagosomes. (f) Proteases within the endosome degrade proteins into peptides. (g) The endosome fuses with the Golgi to form the trans-Golgi. (h) Here, the IC is cleaved and released from the MHC class II molecule. This allows the binding of peptides within the endosome to the peptide-binding cleft of the MHC molecules. An MHC-class-II-like molecule (HLA-DM) binds to MHC class II molecules to facilitate the release of the IC (not shown). (i) The MHC class II–peptide complex is then transported to the cell surface of the APC for (j) recognition by the T-cell receptor (TCR) of (CD4+, CD3+) T-helper lymphocytes (THLs) and (k) intracellular signalling for activation. Recycling of MHC molecules and co-stimulation are not shown (fig004smc).
[19 October 1998: Slight modification to the schematic to show four domains of monomeric CD4]
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