Human Leukocyte Antigen (HLA) refers to 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. HLA is essential for the immune response, as it presents pieces of proteins (antigens) to T cells, enabling the immune system to identify and attack pathogens. 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, the recipient's immune system may reject the transplanted organ. Understanding HLA compatibility is vital for successful transplants and for studying autoimmune diseases, where the immune system mistakenly attacks the body's own cells.