I'm a biology teacher and a student recently asked me a question that stumped me. It's an obvious one and it's about a fact I'd always taken for granted.

Textbooks tell us that people who are blood type A will have anti-B antibodies in their blood. The opposite is true for those with blood type B. Blood type O have both anti-A and anti-B antibodies.

Where do these antibodies come from if we have never been exposed to the corresponding blood type? How does the body know to produce e.g. anti-B antibodies if it has never encountered B-antigens before?

Wikipedia tells me we develop them in the first few years of life following exposure to bacteria and food. This just raises further questions - do bacteria have A and B antigens on their surface? And food???

I can't make my understanding of the science fit with what the textbooks are telling me.

Thanks for your help in advance. I love this site and it has helped me (and my students) many many times.

As far as I can see apparently we do generate these antibodies against antigens we are exposed to from bacteria and food in the first 6 months of life. I don't think it's completely clear how this happens but there are some interesting theories:

http://en.wikipedia.org/wiki/ABO_blood_ … n_theories

This is a really interesting question and I had the same one for a consultant haematologist at our medical school. She could not answer it either. If anyone else knows more about the theories of how such antigen exposure occurs, please let us know!

Nick

Yes it is almost certainly cross reaction with bacterial wall proteins digested and absorbed in the gut. See
http://books.google.co.uk/books?id=68en … mp;f=false

Explains why antibodies only appear after breast feeding (when gut colonisation occurs) and that the natural antigens are enzymes that are also present in bacteria.

http://www.pnas.org/content/early/2012/ … 9.abstract

Nice paper showing we share the same polymorphism that underlies ABO with primates.