ABO antigens are carbohydrates and found on blood cells as well as endothelial cells. They differ from HLA antigens which are glycoproteins. Antibodies to the ABO antigens are not present at birth but usually develop after a few months of life. Those with blood group A do not develop anti A antigens as the B cells that might react to the A antigen are deleted to prevent autoantibodies forming. Similarly blood group B do not produce anti B antigens and blood group AB do not produce either. Those with blood group A develop anti B antibodies, likewise blood group B develop anti A antibodies and blood group O develop both anti A and anti B antibodies.
It is thought that these antibodies are developed in response to the gut colonisation by E coli which has proteins on its coat which closely match the blood group antigens. Antibody strength is measured as a serial dilution - so an antibody measured at 1:16 will cause cell lysis at a dilution of 16 times, adults commonly have antibody measured as 1:512 or 1:1024. The higher the dilution required to achieve cell lysis the stronger the antibody.