A few years ago, I made a comment in a similar topic asking for more details, and I got a very good reply. Hat tip to tait:
> It's complicated.
> There are more than 35 red blood cell groups (see https://www.science.org.au/curious/people-medicine/blood-typ... for a nice writeup). For each of those blood groups, there is more than one possible configuration of some protein or carbohydrate (something like more than one possible genetic sequence leading to more than one kind of molecule on the surface of the RBCs).
> For the other blood groups, I think every case the groups were identified because a patient somewhere made an antibody, causing either a transfusion reaction (if not tested ahead of time) or, more likely, a positive (incompatible) reaction on in compatibility testing.
> It's complicated.
> There are more than 35 red blood cell groups (see https://www.science.org.au/curious/people-medicine/blood-typ... for a nice writeup). For each of those blood groups, there is more than one possible configuration of some protein or carbohydrate (something like more than one possible genetic sequence leading to more than one kind of molecule on the surface of the RBCs).
> And, even with ABO, there can be infrequent variations that make things more complicated (see https://professionaleducation.blood.ca/en/transfusion/best-p... for more).
> For the other blood groups, I think every case the groups were identified because a patient somewhere made an antibody, causing either a transfusion reaction (if not tested ahead of time) or, more likely, a positive (incompatible) reaction on in compatibility testing.
> [...]
It's worth reading the full original comment because it has more interesting details https://news.ycombinator.com/item?id=33507052