At what age do anti-A and anti-B reach adult levels and which immunoglobulin class is prevalent in adults?

ABO antibodies reach adult levels at 5 to 10 years of age.

IgM is prevalent, although most adults would be expected to have some IgG anti-A and anti-B (depending on blood group) due to exposure to A and B antigens via vaccinations. Vaccines can stimulate IgG ABO antibodies as they may contain A- and B-like bacterial antigens or A and B antigens on white cells used to culture the organisms. Group A and B individuals tend to produce low levels of IgG antibodies following vaccination.

Women may have high titres of anti-A and anti-B from exposure to ABO-incompatible fetal cells during pregnancy or at delivery.

Group O people often have more IgG than group A or B individuals as they tend to produce primarily IgG anti-A and anti-B from vaccination.