Major histocompatibility complex (MHC) class II molecules are cell surface proteins that play a crucial role in the immune system by presenting antigens to T cells, which are essential for initiating an immune response. MHC class II molecules are primarily expressed on antigen-presenting cells such as dendritic cells, macrophages, and B cells. They present peptides derived from pathogens or foreign proteins to CD4+ T cells, leading to the activation of specific immune responses. Research in the MHC class II field focuses on understanding the structure, function, and regulation of these molecules, as well as their role in various diseases, including autoimmune disorders, infectious diseases, and cancer. Additionally, MHC class II molecules are important targets for vaccine development and immunotherapy.