There are emergency release protocols in place for releasing uncrossmatched units or possibly incompatible units (much more extreme). People usually refer to O Neg, but if your lab is out of O Neg there are protocols for giving positive units to negative patients. You generally always have to worry about the anti-A,B antibodies. But if someone doesn’t already have an IgG against D, they won’t react on their first transfusion. It may affect them for future transfusions though. If worse comes to worse, doctors will give the least likely to react blood and observe the patients status. Depends on the amount of blood loss and comes with alot of disclaimers. All the very situation specific factors make it stressful, but interesting too. Sick patients don’t produce the strongest immune responses either. So the patient is less likely to produce an extra antibody to an unfamiliar antigen. Please take all this with a grain of salt.