The M-L system is involved in active red cell potassium transport and polymorphisms in this system result in breeds of sheep with varying erythrocyte potassium content. Neonatal isoerythrolysis has been reported in lambs administered bovine colostrum. This is due to the presence of antibodies to sheep erythrocytes in bovine colostrum (called “heterophile” antibodies), which is a common occurrence. They are antibodies produced to common cross-reactive antigens present on the surface of bacteria and protozoa that are identical to epitopes on blood group antigens.