ABO incompatibility is caused by a mismatch between the A or B antigens on red blood cells and the anti-A or anti-B antibodies already present in a person's plasma.
You inherit your ABO type from your parents, and your immune system naturally makes antibodies against the antigens your own cells lack, which is why type O plasma carries both anti-A and anti-B. The mismatch only matters when the two meet: during a transfusion of the wrong red cells, or during pregnancy and birth, when antibodies from a type O parent cross the placenta and reach a type A or B baby. The pregnancy form is common and usually shows up as newborn jaundice in the first day or two of life, and it is treated with feeding support and phototherapy when bilirubin runs high. A transfusion mismatch is rare because of repeated blood typing and bedside identity checks, but it is a medical emergency when it happens. Any newborn who looks yellow in the first 24 hours, or any person who feels chills, back pain, or shortness of breath during a transfusion, needs a clinician right away.
What ABO incompatibility means
Common questions
What causes ABO incompatibility?
ABO incompatibility happens when blood carrying A or B antigens meets plasma that holds antibodies against those antigens. You inherit your ABO type from your parents, and your body makes antibodies against the antigens you do not have, so type O plasma contains both anti-A and anti-B. Two situations bring them together. One is a transfusion of mismatched red cells, almost always traced to a labeling or patient identification error. The other is pregnancy, when a type O parent carries a type A or B baby and antibodies cross the placenta. The pregnancy form is common and usually causes mild newborn jaundice rather than serious illness.