The original RhoGam studies were done in Rh negative prison volunteers at Sing Sing prison in New York. They wanted to give the volunteers IV infusions of Rh positive blood and then come back the next day and give them Rh immune globulin. The warden would not allow them to do that because of prison security considerations so they had to wait to come back in three days to see the same prisoners, thus the 72 hour wait time. I learned this in SBB school a long time ago and still remembered it because it seemed so unscientific.
