Blocked D phenomenon.

Abstract

The blocking of D antigen sites by IgG anti-D in severe cases of Haemolytic Disease of the new born (HDN) is not a new phenomenon1. The coating maternal anti-D prevents the agglutination of the D positive red cells (RBC) by the IgM typing reagent anti-D. Blocking tests were actually the first one used to prove the existence of non-agglutinating IgG antibodies1–3. However the total blocking of D antigen sites by IgG resulting in false negative D typing when a human IgM anti-D was used are rare. In Issitt’s 1981 book, Applied Blood Group Serology, 2nd edition, he says that he surveyed the staff in his own laboratory and found, that even workers having over 30 years of work experience between them, had identified only four cases of blocking anti-D. Issitt4 commented that this phenomenon, much loved by teachers of immunohaematology, is far more likely in theory than encountered in practice. Where it does occur, the anti-D does not necessarily have to be of a high titre. Sulochana et al.5 described a case of blocking anti-D in 2008 with a maternal IgG anti-D titre of 32, the IgM titre was 1,024. In this case the maternal and baby’s RBC were initially grouped as B RhD negative in the local hospital. Due to profound jaundice and signs of kernicterus the baby was transferred to the neonatology department of the Medical College Hospital. Despite three exchange transfusions with B RhD negative blood the Direct Antiglobulin Test (DAT) remained positive and free anti-D was still detectable in the baby’s plasma. Anti-D with a titre of 32 was eluted from the baby’s RBCs. Antenatal grouping and atypical antibody screening had not been performed so an early opportunity to both detect and quantify the anti-D had been missed. This would have informed on monitoring and preventative management through the pregnancy. In this issue of “Blood Transfusion” Verma et al.6 describe a case of blocked D in RhD haemolytic disease of the foetus. At 20 weeks gestation the maternal anti-D titre was found to be 256 by conventional tube technique. Subsequent ultrasound screening showed the foetus to be hydropic and percutaneous umbilical blood sampling confirmed the findings of foetal anaemia (Hb 54 g/L and haematocrit 13.9%). The foetal RBC grouped as RhD negative with a 4+ DAT, an eluate yielded anti-D, showing the D typing to have been blocked. Successful intrauterine transfusion (IUT) was performed in that post transfusion the Hb had incremented to 141 g/L and Hct 41.8%. An icteric baby was delivered, again grouping as RhD negative, but on this occasion due to the ORhD negative blood used for the IUT(s). The blocking phenomenon is not limited to anti-D. Other blood grouping failures have been reported e.g. two cases of false negative K1 typing of foetal cells due to blocking maternal IgG anti-K7,8. Lee et al.7 reported on a case of K1 blocking and showed evidence that various antenatal anti-K1 samples with a titre of 256 or greater can exhibit the blocking of K1 antigens. The number of K1 antigen sites per RBC is in the range 4,000–18,000. The author has also shown the blocking of Fya antigen sites with high titre HIMA-19 (human-murine) anti-Fya in a simulated experiment (unpublished observation). False negative typing results caused by potent maternal IgG antibodies blocking antigen sites are not common when using modern monoclonal blood grouping reagents, especially anti-D. With the immunogenicity of D antigens being only second to ABO antigens, accuracy in RhD typing is critical in transfusion medicine. British Committee for Standards in Haematology (BCSH)9 antenatal grouping and screening guidelines provide guidance on the most secure way to identify potentially harmful cases of HDFN, the outcomes of which may include evidence of RhD antigen blocking. The performance of a cord DAT is also advocated. The performance of an eluate on subsequent DAT positive samples and antibody identification panel will provide the answer to both the causative antibody and the antigen expression of the neonate’s RBC.