I imagine that donations will continue to be important for a range of subtler purposes, such as using the plasma for immunological stuff.
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.