If you have a common blood type, then the fraction of people in need who share your type is higher too. A lot of people need what you have.

If you have a rare blood type, the fraction of the donor population available to supply it is lower. Those who need what you have don’t have as many willing to give it.

Either way, your type is needed!

Yes, you don’t always need a perfect match—A- can give to A+, for example. But since O- can give to anyone, it’s used for when there is no time to perform a matching test to see if the transfusion is safe. So to avoid depleting O-, they’ll give the B+ person B+ instead of O-, even though both would have worked. I figure that type of triage logic also applies to, say, deciding whether to use B- on the B+ patient. And having your blood be used so a different type is available to be used on another also plays its role in saving lives.

I’d imagine the logic for plasma is the same, just with reverse eligibility, where AB+ is the universal donor and O- the universal receiver.

Now if you give platelets… they are pretty much always in demand no matter the type. Short short shelf life.

(Posted something like this before, but most of my posts from Mbin, including that one, went down. So reposting it!)