Table 2. Donor types for allogeneic hematopoietic cell transplantation by HLA status.

	Related donors			Unrelated donors
	Monozygotic twins	HLA-identical siblings	Haploidentical family	HLA-compatible	Partially HLA-compatible	Umbilical cord blood
Major antigens	0	0	≤ 6	0	1–5	0–5
Minor antigens	None	+	+	++	++	++
Probability	< 1%	25%	> 90%	75%	> 90%	> 90%

Unrelated donors: Stem cell donors who are not family members; haploidentical family: difference for an entire HLA haplotype, i.e., for one of the two chromosomes carrying the six HLA loci (HLA-A, -B, -C, -DR, -DQ, -DP); HLA-compatible: shown is the possible number of differences for the five most relevant HLA loci (HLA-A, -B, -C, -DR, -DQ); in around 80% of cases there is also a difference for one or two HLA-DP alleles (21); major antigens: total number of HLA differences; minor antigens: polymporphic peptides encoded by variable genes different in donor and recipient (11);their number is lower in related (+) than in unrelated (++) donors owing to the greater genetic similarity between related than between unrelated individuals. Probability: the likelihood of identifying at least one of the respective types of donors for a patient in Germany; the percentages are based on the experience of German transplant centers and may vary according to ethnicity; HLA, human leukocyte antigens