OK, it seems that almost all the response to the ending of the book is focused on David's assertion that AB- is the universal blood type. I'd discussed this many times throughout the story, but since it's so counter-intuitive, I guess it really needs to be discussed in detail each time the subject is broached.
Plasma and whole blood blood-types are polar opposites. That's because red blood cells absorb antibodies, leaving the whole blood free of those antibodies. Thus if you are donating blood, Type "O" is the universal blood type, even though type O is loaded with antibodies. But plasma and whole blood are different.
I'd almost finished writing the story when an editor suggested I reexamine blood types and plasma donation, which proved to be difficult, since plasma usage was largely abandoned by the US, meaning there's now very little documentation about plasma in the literature nowadays. I had to scramble to change all the references to blood types, and during the revision I tried to explain the differences between them, but of course, since it runs counter to what people are told each time they donate blood, a couple brief explanations are really sufficient.
Looking at the segment in chapter 20 again, I realized that the way it was written there isn't really room for David to explain the differences between whole blood and plasma as she discusses what he's planning to do in the future, but I've tried to adjust it to make it a little clearer. It now reads:
"No, Alice," David responded, directing the answer directly to his own daughter, "you and I can't travel everywhere. As the only type 'AB-' survivors, we're the only universal [plasma] donors, and thus our blood is too valuable. Since our [plasma] can help everyone, we need to stay in a central location so we can supply whoever needs more plasma." David paused, a wicked smile growing on his lips as he reviewed each of them, especially those he'd treated. "However, the others, with their various blood types, will set out for different cities. They can donate blood plasma on their own, assuming they can find the medical supplies. Tom already assures me he can hook them up to run off a car's power supply as long as the car remains stationary. We'll give them as much of our universal blood as we can, as well as a cross sampling of the other blood types, as much as we can manage.
Since it doesn't discuss how plasma differs from whole blood, it's unlikely to prevent confusion, but it's the best I can think of without rewriting David's whole discussion of his future plans.