Can I get your opinion or insight on this? Thank you.

[SbbPerson ★]

It has been over 100 years since the ABO blood types have been discovered, but we still don't really know what are their "function"? What exactly do they do? What did they evolve from?  Thank you

[Malcolm Needs ★]

In terms of the function of the various ABO blood types, there have been a huge number of peer-reviewed papers written on the subject (and the number has exploded with the advent of COVID19).  I would seriously defy anyone to keep up with all of these, but I would recommend reading pages 42-43 of Reid ME, Lomas-Francis C, Olsson ML.  The Blood Group Antigen FactsBook.  3^rd edition, 2012. Academic Press.  ISBN: 978-0-12-415849-8.

In terms of how they evolved, it is so far back now that it is anyone's guess, but slides 28 to 32 of the attached lecture may give you some idea.

In Depth Lecture on The ABO and H Blood Group Systems.pptx

[Mabel Adams]

ABO antigens are present on many other tissues besides red cells.  I wonder if they serve additional functions in some of those tissues.

[Malcolm Needs ★]

14 hours ago, Mabel Adams said:

ABO antigens are present on many other tissues besides red cells.  I wonder if they serve additional functions in some of those tissues.

I cannot see why they would be expressed as histoantigens if they did not have a function, but, apart from causing problems with some organ transplants, I am a bit foxed as to what the function may be.