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 HLAs between donors and recipients can reduce the risk of transplant rejection. There are several classes of HLAs, with Class I and Class II being the most significant in immune responses.