Blood Group: ABO
Common: Yes, especially among blacks. 53.6% of black type O people have anti-A
Causes HDN: Yes
Critical Titer: 1:16
One of the important things to note is that there is a difference in the severity of HDN based on race. HDN due to anti-A or anti-B is more severe in blacks and latinos than in whites [2, 3]. It is possible to have both anti-A and anti-B at the same time . It is especially important that black infants are not discharged from the hospital prematurely and without proper follow up when ABO incompatibility is suspected. It has been suggested that a 6 hour bilirubin can determine the risk for significant hyperbilirubinemia (4 mg/dL) or severe HDN (6 mg/dL) due to ABO incompatibility .
ABO HDN remains the most frequent cause of severe and early jaundice in newborns . Treatments include exchange transfusion and IVIG in addition to phototherapy. Exchange transfusion can lead to a 44.03% reduction in indirect bilirubin . IVIG can reduce the need for exchange transfusion [4, 5]. One study suggests that maternal titers >1:512 can be good predictors of the need for IVIG and exchange transfusion , while another study says that titers aren’t useful in predicting the need for exchange transfusion .
In general HDN from Anti-B is viewed as being more severe than anti-A. This is because severe HDN due to anti-B is seen more frequently than severe HDN due to anti-A, however anti-A can cause severe HDN too. . Another study said that anti-A and anti-B occur with almost the same frequency and there is no difference in severity .
In 4 of 11 cases of anti-A, the baby’s direct coombs test was negative, but the infant still affected. In the remaining 7 cases, the coombs test was weakly positive. In all 11 cases, the mother’s blood would destroy cells with A antigens in the lab.  This can be explained because there are less A antigen sites on infant cells than adult cells . The reason the direct coombs test comes up negative when it should be positive is because of the lower level of antibodies sensitizing the cells .
12 - http://www.bloodjournal.org/content/8/7/620.short?sso-checked=true
13 - http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2141.1971.tb00782.x/full
14 - http://www.thelancet.com/journals/lancet/article/PIIS0140-6736(49)92132-7/abstract
15 - https://www.ncbi.nlm.nih.gov/books/NBK2267/