Blood Group: MNS
Common: Approximately 10% of the population is SS and capable of forming anti-s.
Causes HDN: Yes
Critical Titer: 1:16*
Anti-S is more common than anti-s, but both can cause severe HDN . Although approximately 10% of the population is SS and capable of forming anti-s, there are very few examples. In one article, there were 3 examples of anti-s to date and the authors reported the fourth case which was fatal at 39 weeks. Despite this rarity, anti-s can be fatal. In all 3 examples, titers were only 1:8. In spite of low titer, anti-s is capable of rapid red blood cell destruction, causing severe HDN.  In another article, the infant had severe HDN and required 2 exchange transfusions . In this 1972 article documenting mild HDN due to anti-s, the titer was 1:16, and the infant had a cord bilirubin of 3.0, yet was not treated . Current practices would have put the infant under phototherapy lights.
*With multiple examples of anti-s causing issue at low titer, it begs the question of if 1:16 is the appropriate critical titer for this antibody.
1 - https://www.ncbi.nlm.nih.gov/books/NBK2274/
2 - https://academic.oup.com/ajcp/article/29/3/254/1768391/Hemolytic-Disease-of-the-Newborn-Resulting-from
3 - http://jcp.bmj.com/content/jclinpath/25/9/772.full.pdf
4 - http://onlinelibrary.wiley.com/doi/10.1111/j.1600-0609.1969.tb01809.x/full
5 - http://onlinelibrary.wiley.com/doi/10.1111/j.1537-2995.1961.tb00022.x/abstract