Class II Major Histocompatibility Antigens, also referred to as Class II MHC Antigens, are proteins found on the surface of certain immune cells that play a crucial role in the adaptive immune response. They belong to a larger family of proteins known as Major Histocompatibility Complex (MHC) molecules.
These Class II MHC Antigens are primarily expressed on the surface of antigen-presenting cells (APCs), including dendritic cells, macrophages, and B cells. They function by binding and presenting antigens derived from extracellular sources, such as bacteria, viruses, or other foreign proteins, to T helper cells.
The structure of Class II MHC Antigens consists of two polypeptide chains, typically an α and a β chain, which are encoded by genes within the MHC locus. These chains combine to form a peptide-binding groove that can accommodate short protein fragments (peptides) derived from the digested antigens.
The binding of peptides to Class II MHC Antigens occurs within endocytic vesicles of the APCs, where antigens are internalized through processes such as phagocytosis or pinocytosis. Once the peptides are loaded onto the MHC molecules, they are transported to the cell surface and displayed to T helper cells.
The recognition of antigens by T helper cells, facilitated by Class II MHC Antigens, triggers the activation of immune responses, including the production of specific antibodies by B cells and the release of cytokines that coordinate various immune processes.
The genetic diversity of Class II MHC Antigens is essential for the ability of the immune system to respond to a wide range of pathogens. Variations in these antigens among individuals contribute to the phenomenon of histocompatibility and influence the
