What type of peptides do MHC-I antigens from unhealthy body cells display?

• A. Normal proteins that are recognized by the immune system
• B. Abnormal proteins that are seen as foreign
• C. Only proteins produced by healthy cells
• D. Peptides from foreign bacteria

Answer: (B)

MHC-I antigens from unhealthy body cells display abnormal proteins that are perceived as foreign by the immune system. When a cell is stressed, infected, or transformed (for example, by cancer), it may produce abnormal proteins resulting from mutations, viral infections, or other stressors. These altered peptides are presented on MHC-I molecules, which play a crucial role in the immune response.

When immune cells, specifically cytotoxic T cells (CD8+ T cells), encounter these MHC-I molecules displaying abnormal proteins, they recognize them as indicators of cellular distress or infection. This recognition prompts the cytotoxic T cells to initiate a response to eliminate the unhealthy cells, thus helping to protect the body from diseases such as infections and cancer.

In contrast, normal proteins from healthy cells would not typically provoke such a response, as these proteins are recognized as self by the immune system, whereas peptides from foreign bacteria are typically displayed by MHC-II molecules, engaging different immune pathways.