The HLA antigen is a marker on the cells and the combination of these determines which HLA type you have. On the surface of most cells are markers called HLA (Human Leukocyte Antigen), tissue type or tissue antigen. The HLA antigen exists in a very large number of variants and can be a barrier during transplantation. The HLA type is something you inherit from your parents and is now available in millions of different combinations. On average, the chance of two siblings having the same HLA is 25%.