[nick238]

There are 48 blood type systems, of which ABO (giving A, B, AB, and O) and Rh (+/-) can be combined to form the 8 common types.

There are effectively millions of types because all the systems combined combinatorially, but most antigens beyond ABO and Rh don't cause that much of an issue, so in emergency cases, they just go with them.

[gus_massa]

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).

> 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

[thaumasiotes]

Why are A and B considered to belong to the same "system"? They combine with each other combinatorially in exactly the same way that rhesus factor combines with them, and presumably the same way that all other systems combine with all other systems.

[hn_throwaway_99]

Blood type systems are defined by the single allele that encodes the antigens (as you point out, sometimes multiple antigens per allele). This table shows all of the different blood type systems, https://www.isbtweb.org/resource/tableofbloodgroupsystems.ht..., and the chromosomal location of the respective allele.

[wbl]

ABO all involve the same gene locus and the same protein just different glycans that get added. https://en.m.wikipedia.org/wiki/ABO_(gene)

[thechao]

Hold my beer; I'm gonna middlebrow this! My best guess (dimly remembered from drawing blood for testing in my lab) is that these "groups" (systems?) all live at the same place on the chromosomes that do/n't express them — they're alleles.

[hinkley]

Is that what’s going on with organ matching?

[thecrims0nchin]

Abo is a major part of organ matching. Abo has to be considered for any organ transplant. Every organ differs but HLA antigens is a different, more complex set of antigens that needs to be matched to some degree as well. Abo is usually a simple test, so hla antigen matching is most of what "organ matching" labs spend their time on. I work in this field.

[volemo]

Yes, but even more complicated.