Rhesus disease only happens when the mother has rhesus negative blood (RhD negative) and the baby in her womb has rhesus positive blood (RhD positive). The mother must have also been previously sensitised to RhD positive blood.
Sensitisation happens when a woman with RhD negative blood is exposed to RhD positive blood, usually during a previous pregnancy with an RhD positive baby. The woman’s body responds to the RhD positive blood by producing antibodies (infection-fighting molecules) that recognise the foreign blood cells and destroy them.
If sensitisation occurs, the next time the woman is exposed to RhD positive blood, her body produces antibodies immediately. If she's pregnant with an RhD positive baby, the antibodies can cross the placenta, causing rhesus disease in the unborn baby. The antibodies can continue attacking the baby's red blood cells for a few months after birth.