HLA, or Human Leukocyte Antigen, is a group of proteins found on the surface of cells that play a crucial role in the immune system. These proteins help the body recognize which cells are its own and which are foreign, such as bacteria or viruses. By presenting pieces of these foreign invaders to immune cells, HLA helps trigger an immune response to protect the body from infections. HLA is highly variable among individuals, which is why organ transplants can be challenging. If the HLA proteins of the donor and recipient do not match closely, the recipient's immune system may reject the transplanted organ. Understanding HLA compatibility is essential for successful transplants and can also be important in certain autoimmune diseases.