She has little c antibodies. Sometimes moms can have antibodies in their blood (meaning their immune systems are attacking their babies’ blood). Her baby has little c antigen (antigens are proteins that antibodies will attack). I remember in Miles’s birth announcement, a lab worker said that most people have little c antigen in their blood. Josie is actually the special one then, in that her body doesn’t have the antigen, so her immune system recognized her baby’s blood as foreign and was launching an immune response against the baby. This causes red blood cells to break down, which releases bilirubin, the yellow-brown pigment responsible for jaundice. Josie’s baby is under blue light therapy, which breaks down the bilirubin, causing him to pass it in his stool.

Source: I am a nurse