Blood Group: Duffy
Common: 34% of caucasians, and 22% of blacks, are Fya- and at risk for developing anti-Fya. The duffy null phenotype is very rare in caucasians but found in 68% of blacks who would be at risk for developing anti-Fya. 
Causes HDN: Yes
Critical Titer: 1:16
HDN due to anti-Fya is uncommon and usually mild. In a study of 68 affected pregnancies, only 3 were severely anemic, and 2 of those required IUTs . In another article, 3 of 18 Fya positive infants required phototherapy, while only one required an exchange transfusion . This stands in contrast to a French infant who required 4 IUTs and an exchange transfusion after birth  and a literature review saying that 32% of anti-Fya cases required an exchange transfusion . Additional cases of anti-Fya requiring IUTs have been recorded .
Anti-Fya can have a negative direct coombs test in the infant but baby still be affected .
As of 2007, Ohio State University concluded that anti-Fya has “the potential to lead to significant fetal hemolysis. Management guidelines developed for D sensitization are appropriate for pregnancies complicated by anti-Fy(a) alloimmunization.” 
1 - http://onlinelibrary.wiley.com/doi/10.1002/(SICI)1097-0223(199906)19:6%3C533::AID-PD570%3E3.0.CO;2-5/full
2 - http://onlinelibrary.wiley.com/doi/10.1046/j.1365-3148.1997.d01-38.x/full
3 - http://onlinelibrary.wiley.com/doi/10.1111/j.1423-0410.1959.tb04027.x/full
4 - http://www.tandfonline.com/doi/abs/10.3109/00016349509021175
9 - https://www.ncbi.nlm.nih.gov/pubmed/260466210 - https://www.ncbi.nlm.nih.gov/pubmed/885330