Yes, that is true. The baby will not always die though, but there is a chance. If your blood does contain the protein, your blood is said to be Rh+. If your blood does not contain the protein, your blood is said to be Rh-. Sometimes, a baby will inherit an Rh+ blood type from its father while the mother has an Rh- blood type. This is when the baby's life could be in danger if the mother has Rh- blood because the negative blood attacks the baby's Rh+ blood. If this happens, an exchange transfusion may save the baby's life. The baby's blood can be exchanged for new blood that matches the mother's. I have also heard the mother's blood only attacks the baby blood 13% of time, but I don't know how accurate it is. But if a Rh- woman who has been exposed to Rh+ cells in her first pregnancy gets pregnant again, her body "remembers," and starts producing antibodies when exposed again to Rh+ cells. These antibodies can cross the placenta and attack the fetus's RBC. Also, if this is the case, during a woman's second pregnancy she would be given shots of Rhogam at 28 weeks and after the delivery to prevent mixing of the blood.
Hope this is helpful!