The Kell blood group system is complex and contains many antigens that are highly immunogenic. These antigens are the third most potent, after those of the ABO and Rh blood groups, at triggering an immune reaction.
Antibodies that target Kell antigens can cause transfusion reactions and hemolytic disease of the newborn (HDN). In the case of HDN, ABO and Rh incompatibility are more common causes. However, disease caused by maternal anti-ABO tends to be mild, and disease caused by maternal anti-Rh can largely be prevented. The infrequent cases of HDN caused by Kell immunization tend to result in severe fetal anemia because maternal anti-Kell target fetal red blood cell (RBC) precursors, suppressing the fetal production of RBCs.
(Blood Groups and Red Cell Antigens, Chapter 8: The Kell blood group, http://www.ncbi.nlm.nih.gov/books/NBK2270/ )