Rhesus antigen (preventing reaction to): Rhesus antigen is a chemical that is found on the surface of red blood cells in some people. If a foetus has rhesus antigen and the mother does not and if foetal blood somehow gets into the mother's bloodstream then the mother's immune system will start to attack the foetus's red blood cells. This can lead to complications that may affect the baby after birth, or complications with a different pregnancy at a later stage should the mother become pregnant again.