Final answer:
MHC class II molecules present exogenously derived peptides from extracellular pathogens to T cells, and are found only on professional antigen-presenting cells like macrophages, dendritic cells, and B cells.
Step-by-step explanation:
MHC class II molecules present exogenously derived peptides. These are typically extracellular antigens, characteristic of many bacteria, parasites, and fungi, which are not able to replicate within the cell's cytoplasm. Such antigens enter the endomembrane system of cells via receptor-mediated endocytosis, merging with vesicles that contain pre-formed MHC class II molecules. After the fusion of these vesicles and the association between antigen and MHC, the complex is transported to the cell surface. Unlike MHC class I molecules, which present endogenously derived peptides such as those from viruses that replicate inside cells, MHC class II molecules are exclusively found on the surface of professional antigen-presenting cells (APCs), which include macrophages, dendritic cells, and B cells.
Therefore, the professional APCs use MHC class II molecules to present foreign antigens for the initial activation of T cells, playing a crucial role in the adaptive immune response.