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[1,2]: Most people don't have antibodies, so big K+ blood is fine for most people (we worry about avoiding transfusing big-K positive blood into big-K negative people who will be transfused many times, such as somebody with sickle cell disease, it's not really a major issue for most people getting a rare transfusion).
So, K+ people are definitely allowed to donate. Also, there are people who have antibodies to little-k, and so they would actually need big-K homozygous blood.
When we need blood that is big-K negative, such as for a chronically transfused patient or one with antibodies to big-K, we try find units from people whose units were previously identified to be big-K negative. If they don't have any, the donor center can screen blood for them. We then verify compatibility with a crossmatch (reacting plasma from the patient with RBCs from the unit to make sure there is no reaction).
Remember there are lots of other blood types - big-K is one we have to match sometimes. Little-k is another. There are at least 70 altogether, so we can't really even try to match all of them. Even matching C, E, and K is straining the ability of the system to provide special antigen-negative blood.
It will be interesting to see what the long-term applications of lab-grown blood will be. I do worry about rare blood not being as available.