Significance and Result of Antigen Presented by MHC Class I Molecules

All nucleated cells routinely degrade various proteins in their cytoplasm and load peptides from them into the groove of newly synthesized MHC class I molecules to be delivered to the surface of the cell (figure 16.17a). The proteins are said to be endogenous because they originated within the host cell. If a host cell is not infected or cancerous, all the peptides presented by MHC class I molecules will be from normal proteins, which the immune system perceives as "self" (figure 16.18). As we will discuss later, T cells that can recognize "self" peptides are generally either eliminated during T-cell development or are never activated; thus, effector T-cytotoxic cells should not normally recognize these peptides. If, however, a host cell is infected with a virus or a bacterium that resides in the cytoplasm, or if the cell is producing certain abnormal proteins such as those that characterize a cancerous cell, then some of these peptides presented by MHC class I molecules will likely be recognized by effector T-cytotoxic cells. In

Peptide-binding groove

Peptide-binding groove

Peptide-binding groove

Peptide-binding groove

(b) MHC Class II Molecule

Figure 16.16 MHC Molecules (a) MHC class I molecule; cytoplasmic proteins (endogenous antigens) are presented in the groove of these molecules (b) MHC class II molecule; proteins taken in by the cell (exogenous antigens) are presented in the groove of these molecules.

(a) MHC Class I Molecule

(b) MHC Class II Molecule

Figure 16.16 MHC Molecules (a) MHC class I molecule; cytoplasmic proteins (endogenous antigens) are presented in the groove of these molecules (b) MHC class II molecule; proteins taken in by the cell (exogenous antigens) are presented in the groove of these molecules.

T-cell receptor

T-cell receptor

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