Blood Group: Duffy
Common: 17% Caucasians, 9% Blacks, 91% Chinese, are Fyb- and at risk for developing anti-Fyb. The duffy null phenotype is very rare in caucasians but found in 68% of blacks who would also be at risk for developing anti-Fyb. 
Causes HDN: Yes
Critical Titer: 1:16
HDN due to anti-Fyb is rare. I’ve got one article that references it, but I don’t have access to view the article .
2 - http://onlinelibrary.wiley.com/doi/10.1046/j.1537-2995.1987.27487264751.x/abstract