Final answer:
MHC molecules present processed foreign antigens associated with MHC class II molecules on antigen-presenting cells, such as macrophages, dendritic cells, and B cells, to T cells.
Step-by-step explanation:
MHC molecules only present processed foreign antigens from various pathogens. These antigens can undergo post-translational modifications such as phosphorylation and glycosylation, which may occur before they are displayed on the MHC molecules. MHC class II molecules are expressed exclusively on antigen-presenting cells (APCs), such as macrophages, dendritic cells, and B cells. These cells play a crucial role in the immune response by presenting antigens to T cells.
Antigen presentation with MHC II involves internalizing antigens, processing them enzymatically into fragments, and then presenting these fragments on the cell's surface in association with MHC II molecules. This complex process is essential for T cells to recognize and mount an immune response against foreign antigens. The peptides presented by MHC II are typically derived from extracellular sources, such as bacteria, that have been phagocytosed by the APCs.