Tis­sue typ­ing invol­ves labo­ra­to­ry ana­ly­sis of the blood sam­ple or cheek swab taken from a
new­ly regis­tered donor. Various cha­rac­te­ristics are ther­eby deter­mi­ned which as a whole
reve­al a type of “blood group” for white blood cells. The HLA cha­rac­te­ristics A, B, C, DRB1
and DQB1 are espe­ci­al­ly important. Five HLA cha­rac­te­ristics are inhe­ri­ted from each parent.
Sin­ce the­re are seve­ral hundred vari­ants for each of the­se five HLA cha­rac­te­ristics, the­re are
tril­li­ons of pos­si­ble com­bi­na­ti­ons. Even more vari­ants can be dif­fe­ren­tia­ted due to modern
fine-typ­ing methods. Sear­ching for a sui­ta­ble donor is the­r­e­fo­re dif­fi­cult: the pati­ent and
donor can only be regard­ed as “gene­tic twins” if the com­bi­na­ti­ons of vari­ants match.