Human blood cells carry a variety of different surface proteins that determine their ABO blood type, but in addition to classification based on these proteins, human blood can be catagorised according to Rhesus, or Rh, factor. The Rh factor blood type is indicated by the positive (+) or negative (-) that follows the ABO type. People with positive Rh factor blood types have red blood cells that carry the Rh factor protein, and those with negative Rh factor blood types have red blood cells that do not carry the Rh protein.
The Rh factor blood type is inherited. If both parents are Rh negative, then their children will be Rh negative, but if one parent is Rh negative and the other is Rh positive, their child could be either positive or negative. It is also possible, though far less likely, for two Rh positive parents to have an Rh negative child.
Rh factor blood types are a particularly important consideration in blood transfusions. If a person with Rh negative blood is transfused with Rh positive blood, they can have a transfusion reaction, in which their body begins to make antibodies against the Rh factor. A similar scenario can occur during pregnancy if an Rh negative mother has an Rh positive foetus, though this can often be monitored and treated.
Most people have red blood cells that do carry the Rh factor, and so are Rh positive, but the frequency varies between different populations. For example 99% of Native Americans and 99% of those from China, Indonesia, Japan and Madagascar are Rh positive. About 85% of people from Europe and the US are Rh positive, but among the Basque population (an indigenous ethnic group from a region between Spain and France) as few as 65% are Rh positive.