Para-Bombay: A blind spot in blood grouping?

Sir,

Para-Bombay or red blood cell (RBC) H negative secretor individuals, having weak or absent anti-H activity, may remain undetected on routine blood grouping, as is illustrated in the following case.

A 64 year old lady was referred to us, as a suspicion of Bombay phenotype had been raised elsewhere. Two years previously, however, she had been uneventfully transfused with O positive blood. We performed grouping by tube method (using saline and low ionic strength solution at 37 and 4°C, and Coombs antiserum) as well as column method. Forward grouping was negative with anti-A, anti-B and anti-AB antisera and positive with anti-D indicating O positive blood group. Anti- H lectin and serum from a known Bombay group donor showed negative reactions with patient cells signifying absence of H antigen on the patient's red cells. Backward grouping with patient's serum showed 4+ reactions with A group cells, and B group cells, and negative reactions with pooled O cells as well as cells from four out of five O group donors. One unit showed a 2+ reaction in the Coombs phase. An antibody screen using a commercial three cell panel was negative.

The patient's RBCs were then typed for Lewis antigens. This showed Le (a-b+) denoting secretor status. On the basis of these tests, para-Bombay (RBC H negative, secretor) phenotype was assigned. Presence of H antigen on the RBC membrane and in secretions is determined by H and Se (secretor) blood group loci respectively, which code for distinct Fucosyl Transferase (FUT) enzymes- FUT 1 and FUT 2.[12] The classical Bombay group is hh/sese, i.e., lacking both H and Secretor gene function, whereas persons with hh/Sese or hh/SeSe, lack H antigen on RBCs but possess it in secretions and are referred to as para-Bombay secretors or RBC H negative secretors.

Para-Bombay group individuals usually retain some H antigen on RBCs and weak anti-H activity, which is often demonstrable only at 4°C or by using absorption and elution techniques. In our patient, no anti-H activity was demonstrated either by routine techniques or at 4°C. Without the use of anti-H lectin or antiserum, the para-Bombay phenotype would have remained unidentified and the patient, grouped as O.

The reported prevalence of Bombay and para-Bombay phenotypes in Indians is reportedly 1/10,000.[2] However, since anti-H is not routinely used in blood grouping, many cases may remain undetected.

Koda et al., attribute Indian Bombay and para-Bombay phenotypes to an inactivating missense mutation at the H locus, accompanied by deletion at the SE locus in the case of Bombay individuals.[3]

Based on their study, Lin Chu et al., stated that Chinese and possibly other para-Bombay individuals who are secretors may be transfused with compatible (especially pre warmed indirect antiglobulin test compatible) units of normal ABO blood groups when units of Bombay or para- Bombay blood are not available’.[4] Whether this inference applies to the Indian population has not been examined. Hence we recommended autologous transfusion in this case, although a previous O blood group transfusion appeared to have produced no reaction.