Table 1.

The P/GLOB blood groups.

Phenotype	Frequency	Antigen present on RBC	Antibodies in serum
P1	20–90%a	P1, Pk, P	none
P2	10–80%a	Pk, P	Anti-P1
p	rareb	none	Anti-PP1Pk
P1k	rare	P1, Pk	Anti-P
P2k	rare	Pk	Anti-PP1

^a These frequencies differ significantly between different populations. E.g. the frequency of P₁ vs. P₂ are virtually the opposite when Caucasians (80 vs. 20%) and Japanese (20 vs 80%) are compared.

^b While the frequency of this phenotype has been estimated at 1 per million, two population groups, Swedes and Amish people, have significantly higher numbers (e.g. 141 per million in Västerbotten county in Northern Sweden [21]).