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The ABO blood type divides each blood type according to whether they have the "A" and "B" antigen(s) (AB has both, O has none). People also have antibodies against the antigens they don't have (AB has none, O has both), even before they have ever come in contact with those antigens.

Why do people have antibodies against these antigens they have never come in contact with? This isn't the normal situation for the immune system (e.g. for a virus or the blood type rhesus factor)

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The only reasonably satisfying exploitation I've ever heard is that human gut flora contains the A and B antigens and thus antibodies are created in a reaction to that. But I've never found satisfying reference material or sources for that explanation. –  dtech Jan 9 '13 at 2:00
    
I am not sure I understand your question fully. The specific immune system has the potential to make antibodies for lots and lots and lots of antigens it has never come in contact with. Essentially every time you get a cold it is the specific immune system producing antibodies for an antigen that it had not encountered. If it had encountered it before, then you would have memory cells and likely wouldn't get sick. In reality it is amazing that we have so few blood antigens and that we can transfer blood as easily as we do. –  KennyPeanuts Jan 9 '13 at 18:01
    
@DQdlM The difference is: normally you will not have antibodies for antigens you have not come in contact with. E.g. when first encountering a specific type of influenza you will start making antibodies against it. This is why vaccination works. But blood antibodies are special: you already have antibodies for blood antigens even if you've never come into contact with other blood. –  dtech Jan 9 '13 at 22:19
    
interesting. So the antibodies are present in the blood even w/o exposure to the antigen? I always assumed that we just had the potential to make the antibodies if the antigen was encountered. I will have to look into this a little more. –  KennyPeanuts Jan 10 '13 at 1:11
    
It is written in "Biology - Campbell and Reece " that development of the antibodies occur due to microbes living in our body having antigens similar to other blood group's antigens. –  biogirl Jun 8 '13 at 11:31

2 Answers 2

up vote 13 down vote accepted

According to the Wikipedia entry for the ABO blood group system:

Anti-A antibodies are hypothesized to originate from immune response towards influenza virus, whose epitopes are similar enough to the α-D-N-galactosamine on the A glycoprotein to be able to elicit a cross-reaction. Anti-B antibodies are hypothesized to originate from antibodies produced against Gram-negative bacteria, such as E. coli, cross-reacting with the α-D-galactose on the B glycoprotein.

The cited reference is Van Oss, CJ (2004) Letter to the Editor: "Natural versus Regular Antibodies The Protein Journal 23:357, available here. This source contains this statement:

..."we have known for more than four decades that these bloodgroup (antibodies) arise out of minor infections occurring very early in life..."

The cited references are:

Pettenkofer et al. Z. ImmunForsch. 119: 415-429

Springer, GF (1960) Klin. Wschr. 38: 513-514

Unfortunately I have access to neither of these articles, and besides they are presumably written in German.

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We each inherit either A, B, AB or no antigens from our parents.

The current thought is that when you're between 0-6 months old you are exposed to bacteria/viruses that contain very similar antigens (A or B). These antigens are similar enough to the A and B antigens found on red blood cells that any antibody created against these bacterial antigens would also react against the corresponding red blood cell antigen. Seeing as you can't produce an antibody against your own antigen (except in rare circumstances), you always produce the opposite antibody to what antigen you have.

That's why sometimes a person with AB antigens will only develop A antibodies, or B antibodies, because they may never have come into contact with one of the antigens.

I hope some of that makes sense. Feel free to point out any glaring mistakes I've made, wouldn't be the first time! ;-)

Source: I'm a biomedical scientist.

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