Serology
Anti-A and anti-B antibodies (called isohaemagglutinins), which are not present in the newborn, appear in the first years of life. They are isoantibodies, that is, they are produced by an individual against antigens produced by members of the same species (isoantigens). Anti-A and anti-B antibodies are usually IgM type, which are not able to pass through the placenta to the fetal blood circulation. O-type individuals can produce IgG-type ABO antibodies.
Read more about this topic: ABO Blood Group System