Blood Group: Kell
Common: Very rare. ~100% of people have Jsb antigen.
Causes HDN: Yes
Prior to 1995, anti-Jsb had not been known to cause HDN. Doctors reported a patient with high anti-Jsb titers causing hydrops and death in one pregnancy, followed by a pregnancy treated with multiple IUTs.  In one case, they treated the baby with IUTs with blood donated from the mother . Anti-Jsb can also be treated with erythropoietin (commonly called Epogen) .
Anti-Jsb has been known to cause anemia requiring transfusion. This happened in a 6 day old infant who was transfused with blood from his mother in the absence of a suitable donor.
It is difficult to severity with anti-Jsb off of titer. In 2001 a patient presented unexpectedly with severe HDN requiring urgent exchange transfusion. Signs of fetal distress were found at 35 weeks. Despite a previous Jsb+ baby who was unaffected, this pregnancy was severely affected. 
3 - https://www.ncbi.nlm.nih.gov/pubmed/11555475