There is a leakage of red blood cells and some of the red blood cells can be exchanged between the fetus and Arab woman. Now, we know that because the woman is RH negative that means within the bloodstream the woman will not have any antibodies against antigen D because that woman was never exposed to antigen D before that. And so she is RH negative. But because the child, the fetus is RH positive inside their blood they do have red blood cells that contain antigen D. And when during childbirth some of these red blood cells of the fetus leak into the blood system of the mother, some of these antigen D red blood cells end up in the mother's bloodstream. And now, because the mother is exposed to antigen D for the first time, the immune system will kick in and begin producing antibodies against the antigen D found on those red blood cells that essentially leak into the mother from the fetus. Now this will not actually affect this fetus in any way because this process takes place during and after childbirth.