Blood Group: Kell
Common: Very rare
Causes HDN: Yes
Anti-Ku can cause severe HDN requiring IUTs. Recombinant erythropoietin (Epogen) has been used successfully on infants affected by anti-Ku .
"The Kell system has a rare null phenotype, Ko, in which RBCs lack all Kell antigens. Individuals with this phenotype are healthy but produce anti-Ku when they encounter RBCs that do express Kell antigens. Anti-Ku is capable of causing a mild to severe transfusion reaction with at least one fatal case being reported (5). Therefore, if Ko individuals ever require a blood transfusion, they should only be transfused with Ko blood products." 
1 - https://www.ncbi.nlm.nih.gov/books/NBK2270/
2 - http://www.tandfonline.com/doi/abs/10.1080/08880010601001453
3 - https://www.ncbi.nlm.nih.gov/pubmed/16271105