The Human Leukocyte Antigen (HLA) test, also known as tissue typing or HLA typing, is a blood test used to identify specific proteins on the surface of cells that stimulate the body鈥檚 immune response. These proteins, or antigens, play a crucial role in the body鈥檚 ability to recognize and differentiate its own cells from foreign substances, such as bacteria, viruses, or transplanted tissue.