Blood for transfusion needs to be crossmatched against antigen types of the recipient. Many patients will tolerate several transfusions of a minor mismatched antigen before developing a sensitivity. Major antigens are what cause significant reactions that can be life-threatening.
Minor antigens come into play when crossmatching for infants and premies, but this is way beyond my scope.
Depending on the severity of the reaction, blood will either be stopped or the patient will be loaded up with Benadryl and Tylenol with the blood running at a slower rate.
I’m reminded of that American high schooler in Uganda running an orphanage and ran into this exact issue when doing a transfusion on a malnourished infant. [1]
She was skilled enough to perform a transfusion and knowledgeable enough to test for a ABO+/- match but not so knowledgeable as to be sensitive to this issue with disastrous results.
On the other hand her clinics metrics were on par or slightly above the local hospitals so it’s not clear to me they would have faired better getting care elsewhere there.
[1] https://stories.showmax.com/za/hbos-docuseries-savior-comple...
We don't actually express antibodies to antigens until we're exposed to them, so crossmatching won't detect a minor antigen mismatch until the first transfusion containing the antigen is administered.
That first time causes a delayed hemolytic transfusion reaction, which is generally milder than the kind of reaction crossmatching will prevent, but can be serious or even fatal.
https://en.wikipedia.org/wiki/Delayed_hemolytic_transfusion_...
Don't we have synthetic blood, at least capable of transporting oxygen ?