In foals, the condition results when a foal inherits red blood cell antigens (which the dam does not have) from its sire. Previous exposure of the mare to these antigens during a previous pregnancy or whole blood transfusion leads to the mare producing alloantibodies to the foal's red blood cells. At birth the foal ingests large numbers of antibodies in the colostrum, leading to severe haemolytic disease. During pregnancy however, the foal is unaffected because blood and antibodies are unable to cross the placenta. | In foals, the condition results when a foal inherits red blood cell antigens (which the dam does not have) from its sire. Previous exposure of the mare to these antigens during a previous pregnancy or whole blood transfusion leads to the mare producing alloantibodies to the foal's red blood cells. At birth the foal ingests large numbers of antibodies in the colostrum, leading to severe haemolytic disease. During pregnancy however, the foal is unaffected because blood and antibodies are unable to cross the placenta. |