Class II antigens, also known as major histocompatibility complex II (MHC II) antigens, refer to a group of cell surface proteins that play a crucial role in the immune response in vertebrates, including humans. These antigens are involved in antigen presentation, which is a vital step in the activation of T cells and the subsequent immune response against pathogens.
Class II antigens are primarily found on antigen-presenting cells (APCs), such as macrophages, dendritic cells, and B cells. They are responsible for presenting antigens that have been internalized by the APCs to helper T cells. This process is crucial for the recognition of foreign substances, such as bacteria or viruses, by the immune system.
The structure of class II antigens consists of two main chains: alpha and beta. These chains combine to form a heterodimer that is embedded in the cell membrane. Each of these chains consists of two domains that are crucial for antigen binding and presentation.
Upon internalizing an antigen, the antigen-presenting cell processes it into shorter fragments. These fragments then bind to the class II antigens, forming a complex that is subsequently recognized by specific T cells. Through this mechanism, class II antigens play a central role in initiating and coordinating the immune response against foreign invaders.
In summary, class II antigens are cell surface proteins involved in antigen presentation. They are primarily found on antigen-presenting cells and play a critical role in the activation of helper T cells, which in turn initiate an immune response against pathogens.
