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 molecules present pieces of proteins from inside the cell to T cells, which are essential for initiating an immune response. There are several different types of HLA genes, and they are highly variable among individuals. This genetic diversity is important for the immune system's ability to respond to a wide range of pathogens. HLA typing is often performed before organ transplants to ensure compatibility between donors and recipients, reducing the risk of rejection.