Final answer:
Class I MHC molecules are primarily involved in the presentation of endogenous antigens to cytotoxic T cells, vital for the immune system to detect and destroy infected or cancerous cells. They are found on all nucleated cells.
Step-by-step explanation:
The primary role of Class I MHC (major histocompatibility complex) molecules is the presentation of endogenous antigens. These molecules are found on the surface of all nucleated cells and present antigens that are derived from the cell's internal processes, such as viral or bacterial proteins that are synthesized inside the cell. This feature allows effector T cells to recognize and respond to cells that have become infected or cancerous. Hence, Class I MHC molecules play a critical part in the cellular immune response by displaying these inner cell-derived antigens to cytotoxic T cells. This is different from Class II MHC molecules, which are found only on professional antigen-presenting cells (APCs) like macrophages, dendritic cells, and B cells and present antigens that have been engulfed from the extracellular environment.