Donation to get antigens for the RH group to give to RH- mother after a RH+ baby is an important application. For example https://edition.cnn.com/2018/05/11/health/james-harrison-blo...
The donation is to get antibodies.
Antigens are are the stimulus that causes the body to form antibodies.
We give the Rh- mom antibodies so they bind to antigens on the blood from baby; the idea being that "hiding" the antigens from mom's immune system will prevent her from making her own antibodies.