HLA antigens, or Human Leukocyte Antigens, are proteins found on the surface of cells that play a crucial role in the immune system. They help the body recognize which cells are its own and which are foreign, such as bacteria or viruses. This recognition is essential for the immune response, as it allows the body to target and eliminate harmful invaders. There are several classes of HLA antigens, categorized into Class I and Class II. Class I antigens are present on almost all nucleated cells, while Class II antigens are mainly found on immune cells like B cells and T cells. The diversity of these antigens is important for tissue compatibility in organ transplants.