Tissue typing is an elaborate method of laboratory analysis that is usually performed on a blood sample. Various characteristics are thereby determined which as a whole reveal a type of blood group for white blood cells. The HLA characteristics A, B, DRB1 and DQB1 are of particular importance. At low resolution, there are up to 50 different forms of each once, and humans usually have two of them – one from each parent. This alone theoretically results in millions of combinations, which with modern fine-typing methods can then be differentiated into many more variants.