Human leukocyte antigens (HLAs) 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. HLAs are highly variable among individuals, which is why they are important in organ transplantation. Matching the HLAs of a donor and recipient can reduce the risk of rejection, making successful transplants more likely. The study of HLAs is also significant in understanding autoimmune diseases and genetic diversity.