The process of Rh isoimmunisation occurs in Rh-negative mothers when they give birth to an Rh-positive foetus from the father. Throughout gestation, the blood of both the baby and the mother is exchanged across the placenta. The mother's immune system recognises Rh-negative blood as foreign from a series of antigens on the blood cells and begins to produce antibodies against them.
On the mother's first exposure to Rh-negative Rh, IgM antibodies are produced which are large and do not cross the placenta, but the mother will be sensitised to this antigen. Because of this, for foetal involvement and haemolytic anaemia to occur, the patient must have been previously sensitised and it is on a second exposure to the Rh antigen that IgG antibodies are produced, which are those that cross the placenta, causing foetal involvement.
Although Rh isoimmunisation is the most common, there are many other antigens that can cause isoimmunisation. Because of this, all Rh-negative mothers and all pregnant women are tested throughout their pregnancy to see if there are antibodies capable of causing foetal damage.
Focusing on the initial question, it is possible to use Rh-negative donor sperm to reduce the risk of isoimmunisation due to this antigen, as this way we ensure that the foetus will be Rh-negative. However, there are couples who prefer the sperm used to have the same blood group as the future father and this could also be done especially in patients who have not been previously sensitised.
Currently, due to the controls carried out on pregnant women, the administration of the anti-D vaccine in abortions and pregnancies of Rh-negative women has drastically reduced the cases of Rh isoimmunisation.