;Environmental exposure: Anti-A and anti-B antibodies are usually IgM and do not pass through the placenta, but some mothers "naturally" have IgG anti-A or IgG anti-B antibodies, which can pass through the placenta. Exposure to A-antigens and B-antigens, which are both widespread in nature, usually leads to the production of IgM anti-A and IgM anti-B antibodies but occasionally IgG antibodies are produced. ;Fetal-maternal transfusion: Some mothers may be sensitized by fetal-maternal transfusion of ABO incompatible red blood and produce immune IgG antibodies against the antigen they do not have and their baby does. For example, when a mother of genotype OO (blood group O) carries a fetus of genotype AO (blood group A) she may produce IgG anti-A antibodies. The father will either have blood group A, with genotype AA or AO or, more rarely, have blood group AB, with genotype AB. ;Blood transfusion: It would be very rare for ABO sensitization to be due to therapeutic blood transfusion as a great deal of effort and checking is done to ensure that blood is ABO compatible between the recipient and the donor.
Hemolytic disease of the newborn 
Neonatal meningitis
Neonatal hepatitis
Michelin tire baby syndrome
Neonatal diabetes
Shaken baby syndrome
Twin-to-twin transfusion syndrome
Neonatal toxic shock-like exanthematous disease