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)
Answer
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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