From where do MHC-I antigens obtain the peptides they display?

• A. From foreign pathogens exclusively
• B. From normal protein breakdown in healthy body cells
• C. From damaged or diseased cells
• D. From environmental substances

Answer: (B)

MHC-I (Major Histocompatibility Complex class I) molecules primarily display peptides that are derived from normal protein breakdown within healthy body cells. This process is essential for the immune system's ability to monitor cellular health. Cells continuously degrade their internal proteins into peptide fragments via the proteasome and then transport these fragments into the endoplasmic reticulum (ER), where they bind to MHC-I molecules.

Once the peptides are bound to MHC-I, the complex is transported to the cell surface, where it can be presented to CD8+ T cells (cytotoxic T cells). This enables the immune system to recognize and destroy cells that are displaying abnormal peptides, such as those from infected or cancerous cells. By presenting these self-derived peptides, MHC-I plays a critical role in maintaining self-tolerance and the immune system's functionality in distinguishing between healthy and unhealthy cells.

In contrast to other options, MHC-I does not exclusively source peptides from foreign pathogens or only from damaged cells; rather, it routinely presents self-peptides from normal protein turnover, which helps the immune system assess whether a cell is healthy or requires attention.