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.