Why do people have antibodies against other blood types?

Question

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 1

Some bacterial infections can convert the A antigen into a B like antigen causing the production of anti-B antibody. Other diseases such as thalassemia and some blood cancers can also alter the RBC antigen.

Answer 2

Theoretically antibodies are produced against the antigens with whom our bodies come with, when we are young we are exposed to many antigens of bacteria which have near about similar composition like of blood antigen A and B.