MHC class II molecules are proteins found on the surface of certain immune cells, such as B cells, dendritic cells, and macrophages. They play a crucial role in the immune system by presenting processed fragments of proteins from pathogens to CD4+ T cells. This interaction helps activate T cells, which are essential for coordinating the immune response. These molecules are important for recognizing foreign substances, such as bacteria and viruses. The ability of MHC class II to present antigens is vital for the adaptive immune response, allowing the body to remember and respond more effectively to previously encountered pathogens.