Which MHC class is expressed on the surface of antigen-presenting cells and presents foreign antigens?

MHC class II molecules are specifically expressed on the surface of antigen-presenting cells such as dendritic cells, macrophages, and B cells. The primary role of these molecules is to present processed foreign antigens to CD4+ T helper cells, which are essential for coordinating the immune response. MHC class II molecules bind to peptides derived from extracellular proteins, which have been taken up by the antigen-presenting cells through processes like phagocytosis or endocytosis and then processed inside the cell.

This is a crucial mechanism in the adaptive immune response, as the activation of CD4+ T cells leads to the proliferation of these cells and the stimulation of other immune components, including B cells and cytotoxic T cells. In contrast, MHC class I molecules present endogenous antigens and are found on nearly all nucleated cells, primarily presenting to CD8+ T cells. MHC class III is not involved in antigen presentation; it includes various molecules related to immune function, but does not specifically present antigens to T cells. Thus, the correct answer highlights the unique role of MHC class II in the adaptive immune response.