Which blood type could be given as a transfusion for patient with ab blood when the rh factor is unknown?
o+
a+
ab+
ab–

The right answer is AB-.

In humans, the blood group is determined by the substances on the surface of the red blood cells, called antigens. Blood groups are grouped into systems. In the ABO system, there are four possible blood groups: A, B, O, and AB. In the Rh system, the presence or absence of substance "D" on the surface of the red blood cell determines whether Rh is positive (+) or negative (-).

The combination of the two systems allows classification into 8 blood groups: groups: A +, A-, B +, B-, AB +, AB-, O + and O-.

A Rh- subject can only receive Rh- blood, but a Rh + subject can receive Rh + but also Rh- since the latter does not possess rhesus antigen. therefore if we do not know the rhesus of the subjects we can transfuse Rh-blood without risk (Biensur with ABO compatibility).