PA21.1-6 | ABO & Rh Blood Group Systems — Summary & Reflection

REFLECT

Consider a 24-year-old primigravida at her first antenatal booking visit. She is found to be Blood Group O, Rh-negative.

1. What antibodies are already present in her plasma, and what is their immunoglobulin class? Why are they present despite no prior transfusion?
2. Her husband is Blood Group A, Rh-positive. What is the maximum possible Rh risk to this pregnancy and to future pregnancies if no prophylaxis is given?
3. If this patient were instead Bombay phenotype (typing as 'O' on routine testing), how would your clinical management differ?

Write 3–4 sentences integrating the ABO and Rh concepts from this module before proceeding to SDL 2.

KEY TAKEAWAYS

Key takeaways from SDL 1:

• ABO antigens are oligosaccharides built on H substance by A- or B-transferase enzymes. Group O lacks functional transferase → only H antigen present.
• Landsteiner's law: plasma contains IgM antibodies against ABO antigens absent from own RBCs. These are natural — present from infancy without prior exposure, driven by environmental cross-reactive antigens.
• IgM + complement activation → immediate intravascular haemolysis — the hallmark of ABO incompatible transfusion. This is a medical emergency.
• ABO inheritance: A and B codominant; O recessive. Bombay phenotype (Oh): no H substance → anti-A + anti-B + anti-H; routine grouping misleads; only Bombay blood compatible.
• Rh system: D antigen is a transmembrane protein, most immunogenic non-ABO antigen. No natural anti-D — only forms after sensitisation.
• Anti-D is IgG → crosses placenta → causes HDN (extravascular haemolysis, erythroblastosis fetalis, hydrops fetalis in severe cases).
• Anti-D prophylaxis (RhoGAM) prevents maternal sensitisation; must be given to all Rh-negative mothers at 28 weeks and after delivery.
• Kell, Duffy, Kidd systems: cause alloimmunisation in chronically transfused patients; Duffy-null confers P. vivax malaria resistance; Kidd causes delayed haemolytic reactions.
• Agglutination (IgM-mediated) is the principle of direct blood grouping; IgG antibodies require the Coombs test (SDL 5).