Are MHC class I molecules associated with intracellular or extracellular proteins?

MHC class I molecules are primarily associated with intracellular proteins. This association is crucial for the immune system's ability to monitor cellular health and detect infections. They present peptide fragments derived from proteins that are synthesized within the cell, including viral proteins in infected cells and abnormal proteins in cancer cells.

This presentation occurs on the surface of nearly all nucleated cells in the body, allowing cytotoxic T cells (CD8+ T cells) to recognize and respond to any that display abnormal or foreign peptides. This mechanism is fundamental for the identification and elimination of infected or malignantly transformed cells, ensuring proper immune surveillance.

The other answer choices indicate associations that do not align with the function of MHC class I molecules. Extracellular proteins are typically presented by MHC class II molecules, which are designed to interact with CD4+ T helper cells and are restricted to professional antigen-presenting cells. The notion that MHC class I molecules would associate with both or neither intracellular or extracellular proteins misrepresents the established understanding of MHC class I function in the immune response.