Another role of intracellular protein degradation is related to the immune system by:

• A. Degrading foreign proteins to present to the immune system.
• B. Generating antibodies directly.
• C. Recycling mitochondria.
• D. Producing cytokines.

Answer: (A)

Intracellular protein degradation helps the immune system by sampling what’s inside the cell and presenting those samples to immune cells. The proteasome chops proteins, including foreign or abnormal viral proteins, into peptide fragments. These peptides are transported into the endoplasmic reticulum by TAP and loaded onto MHC class I molecules. The peptide–MHC I complexes are then displayed on the cell surface, where cytotoxic T cells patrol for non-self peptides. When a T cell recognizes a foreign peptide, it becomes activated and can kill the infected cell, helping to control intracellular infections. This is why degradation of intracellular proteins is linked to immune presentation. Antibodies aren’t produced directly from this degradation process, mitophagy isn’t about antibody generation, and cytokines are produced by immune cells in response to activation rather than as a direct product of protein breakdown.