Which type of MHC is found on all cells?

The correct answer is that MHC I is found on all nucleated cells. MHC, which stands for Major Histocompatibility Complex, plays a crucial role in the immune system, particularly in the presentation of antigens to T cells. MHC I molecules present peptide fragments derived from proteins synthesized within the cell, allowing the immune system to recognize and eliminate infected or abnormal cells, including those that have undergone transformation, such as cancerous cells.

This widespread expression of MHC I on all nucleated cells is critical for the immune surveillance mechanism, enabling cytotoxic T cells to monitor the health of virtually every cell in the body. In contrast, MHC II molecules are primarily found on professional antigen-presenting cells like dendritic cells, macrophages, and B cells, which are specialized in presenting antigens to helper T cells. MHC III refers to a different class that includes various proteins related to the immune response but is not involved directly in antigen presentation. MHC IV is not a recognized class of MHC molecules. Thus, the distinct role of MHC I in presenting antigens from all cells forms the basis for its classification and importance in the immune response.