Question about Blood type

[남종민]

Hello, every netters.
I have some questions about human blood type.

The relationship bewtween antigen and antibody in blood type is irony.
In the case of O type, there is no antigen(A and B).
In spite of this, O type human blood has antibody for A and B always.

I think that the direct origin of antibody is exogenous...
As a result of coevolution with bacteria...??????

Hmmm.
Anyone can tell me about this, mail to me.

Thanks in advance..