(P-IG-6) A Rare Case of Anti-P in a Twin Pregnancy

Background/Case Studies: Landsteiner and Levine discovered the P blood group system in 1927 after injecting rabbits with human red blood cells (RBCs). The first antigen discovered in this system appeared to be present on all human RBCs. It was named the P antigen (later renamed P1). The blood group is now classified into three related systems; PP1PK(3), Globoside(28), FORS(31) and collection 209.

P is the only antigen of the Globoside system. P is expressed on all red cells except for rare phenotypes p, P1k and P2k (Table 1). Mutations on the gene B3GALNT1 give rise to the P1k and P2k phenotypes, and these individuals produce anti-P.

Study

Design/Methods: Patient medical records and all relevant laboratory serologic work-up reviewed.

Results/Findings: A 36-year-old female of Southeast Asian descent was admitted at 30 weeks gestation with complete vasa previa and a posterior low-lying placenta. Her antibody screen was positive. On antibody identification, the patient’s plasma was reactive with all cells (3+ AHG in PEG) except the auto control. The impression was a strong cold antibody due to reactivity at RT, but PEG pre-warm technique was found to be 2+ reactive as well. Phenotyping was performed, and a P1 negative result suggested that the antibody was related to that system. P1 substance neutralization and RESt (Rabbit Erythrocyte Stroma) adsorption were performed, but the plasma remained reactive. Treatment of plasma with 0.01M DTT revealed that the antibody had an IgG component. High-incidence anti-sera testing showed that patient cells were reactive with anti-PP1Pk but negative against anti-P. Testing with rare reagent cells confirmed the patient had anti-P and was presumed to be the P2k phenotype. All other alloantibodies were ruled out using PEG-adsorption. The anti-P titer was 1:8, however, the significance of this titer is not well established and the patient was followed by serial MCA (middle cerebral artery) dopplers.

Conclusions: While autoanti-P is commonly seen, particularly in paroxysmal cold hemoglobinuria, alloanti-P is very rare. Alloanti-P will react to approximately 99.99% of RBC donors. Alloanti-P is found in the serum of all P1k and P2k individuals. Siblings of such individuals are encouraged to be tested and, if P-negative, donate. Unfortunately, in this case, siblings who were tested were P-positive.

PP1Pk-negative compatible blood was procured through the American Rare Donor Program. In our P2k phenotype patient, there was consideration that anti-PP1Pk antibodies in donor plasma may be incompatible and any fresh, non-frozen units would need washing to ensure anti-PP1Pk removal. Ultimately, the patient had an uncomplicated delivery and blood was not needed perioperatively.

Importance of research: This case review a rare phenotype and alloimmunization causing the production of an antibody that is incompatible with the majority of the blood donor population. There are limited case reports of this phenotype and antibody in literature, so the diagnostic workup and management are critical contributions to our understanding and care of patients, especially during pregnancy.