What causes ABO incompatibility?
Last updated September 7, 2026.
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 to do
- Know your type: Ask for your ABO and Rh results and keep them with your health records.
- Expect the ID check: Before a transfusion, staff should confirm your name, date of birth, and the unit label at the bedside, so speak up if that step is skipped.
- Report transfusion symptoms: Tell the nurse immediately about fever, chills, back or flank pain, dark urine, or trouble breathing during or shortly after a transfusion.
- Watch the first day: Yellow skin or eyes in a baby under 24 hours old needs same-day evaluation and a bilirubin measurement.
- Feed often: Frequent breastfeeding or formula feeding helps a newborn clear bilirubin, and steady wet diapers are a good sign.
- Ask about the next pregnancy: Tell your prenatal clinician if a previous baby had jaundice or needed phototherapy, since testing can be planned early.
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