MHC II （Major Histocompatibility Complex Class II） proteins are a class of cell surface molecules expressed by antigen-presenting cells （such as dendritic cells, macrophages, and B cells）. They are responsible for presenting exogenous antigenic peptides to CD4+ T helper cells, thereby initiating adaptive immune responses. MHC II molecules consist of two transmembrane chains （α and β） that form a peptide-binding groove, which binds and displays antigenic peptides derived from extracellular pathogens or proteins. Their expression is regulated by immune modulators （such as IFN-γ） and plays a critical role in autoimmune diseases, transplant rejection, and infection immunity. The diversity of MHC II is encoded by highly polymorphic genes, ensuring a broad capacity for antigen recognition, making it a core mechanism of the immune system for identifying and eliminating pathogens.