ABO incompatibility occurs when a person receives blood from a donor with a different ABO blood type. The ABO blood group system classifies blood into four main types: A, B, AB, and O. If a person with type A blood receives type B blood, their immune system may recognize the foreign blood cells as harmful and attack them, leading to serious complications. This condition is particularly important in blood transfusions and during pregnancy. If a pregnant woman has type O blood and her baby has type A or B blood, her body may produce antibodies against the baby's blood type, potentially causing health issues for the newborn.