To me, sitting in a blood bank, those photos are organized to reinforce the problem of just how many units you have to screen when someone with a rare blood typ

To me, sitting in a blood bank, those photos are organized to reinforce the problem of just how many units you have to screen when someone with a rare blood type shows up. What you do is take blood from one of about 10 small testing segments and mix it with the recipient's serum. If it clots or hemolyzes, that's bad. That's the most elementary method of crossmatching units, and it's often still done just before sending a unit out, because the blood's there to be tested, might as well.