Blood Groups & Transfusion
Physiology · Blood · lean revision notes
Blood Groups & Transfusion
Blood group antigens are inherited surface glycoproteins/glycolipids on the red cell membrane; their corresponding plasma antibodies decide transfusion compatibility. This is a perennially tested Physiology + Pathology overlap area covering ABO/Rh genetics, compatibility testing, and the immunology of transfusion reactions and haemolytic disease of the newborn (HDN).
Definition & Classification
A blood group system is a set of red-cell surface antigens encoded by a single gene or a cluster of closely linked genes. Over 40 systems exist (ISBT recognised), but ABO and Rh dominate clinical practice because their antibodies are clinically the most significant (capable of causing brisk haemolysis).
The two pillars:
- ABO system — determined by the ABO gene on chromosome 9. Defines A, B, AB and O groups. Antibodies are naturally occurring (no prior sensitisation needed).
- Rh system — RHD and RHCE genes on chromosome 1. The D antigen is the most immunogenic; antibodies are immune (acquired), formed only after exposure.
| System | Chromosome | Antibody type | Antibody class | Naturally occurring? | Causes intravascular haemolysis? |
|---|---|---|---|---|---|
| ABO | 9 | Isohaemagglutinins (anti-A, anti-B) | IgM | Yes | Yes (complement-fixing) |
| Rh (D) | 1 | Anti-D | IgG | No (needs sensitisation) | Mostly extravascular |
High-yield: ABO antibodies are IgM, naturally occurring, react best at 4°C, fix complement, and cause intravascular haemolysis. Rh antibodies are IgG, immune, react best at 37°C, cross the placenta, and cause extravascular haemolysis. This single contrast answers a large share of transfusion MCQs.
Genetic & Biochemical Basis of ABO
The ABO antigens are built on the H antigen, a precursor oligosaccharide on the red-cell membrane. The H gene (FUT1, chromosome 19) adds L-fucose to the precursor → forms the H substance.
The ABO gene then encodes glycosyltransferases that add a terminal sugar to H:
- A allele → enzyme adds N-acetylgalactosamine → A antigen
- B allele → enzyme adds D-galactose → B antigen
- O allele → non-functional enzyme → H substance left unmodified (so group O has the most H antigen)
A and B are co-dominant; O is recessive (amorph).
| Phenotype | Possible genotypes | Antigens on RBC | Antibodies in plasma |
|---|---|---|---|
| A | AA, AO | A | Anti-B |
| B | BB, BO | B | Anti-A |
| AB | AB | A and B | None |
| O | OO | H only (no A/B) | Anti-A and Anti-B |
High-yield: Group O has the most H antigen; group AB the least. In the Bombay phenotype (Oh) the H gene is absent (hh), so no H substance is made and A/B antigens cannot be expressed even if A/B genes are present. Such patients have anti-A, anti-B and anti-H, type as "O" on routine grouping, and can be transfused only with other Bombay blood — a classic trap question.
Landsteiner's Law
A person's serum contains antibodies against the ABO antigen(s) absent from their own red cells. Anti-A and anti-B appear by 3–6 months of age (cord blood lacks them), peak around adulthood, and decline in old age. This is why forward (cell) grouping and reverse (serum) grouping are both done — they must agree.
The Rh System & D Antigen
The Rh system has 5 principal antigens: D, C, c, E, e. D is the most immunogenic (about 80% of D-negative individuals form anti-D after a single exposure to D-positive blood). There is no "d" antigen — "d" simply denotes absence of D.
- Rh-positive = D antigen present (≈ 85–95% of Indians).
- Rh-negative = D antigen absent.
- Weak D (Du): weakened expression; such patients are treated as D-positive when they are donors but cautiously as D-negative when recipients in many protocols.
High-yield: Order of Rh antigen immunogenicity (after D): D > c > E > C > e. Anti-D is IgG, does not occur naturally, crosses the placenta, and is central to HDN.
Pretransfusion Compatibility Testing
Compatibility testing prevents reactions by ensuring donor red cells survive in the recipient.
Stepwise flow:
Blood grouping (ABO + Rh) → Antibody screening (recipient serum vs panel cells) → Crossmatch (recipient serum + donor cells) → Issue of compatible unit.
- Forward grouping — patient's RBCs tested with known anti-A and anti-B sera.
- Reverse grouping — patient's serum tested against known A and B cells (confirms forward result via Landsteiner's law).
- Rh typing — RBCs tested with anti-D.
- Antibody screen — detects unexpected (irregular) IgG alloantibodies using indirect antiglobulin test.
- Crossmatch:
- Major crossmatch — donor RBCs + recipient serum (most important; detects recipient antibody against donor cells).
- Minor crossmatch — recipient RBCs + donor serum (less important; donor plasma is small in volume in packed cells).
High-yield: The major crossmatch is the single most important pretransfusion test — donor cells against recipient serum. In an emergency, O-negative packed cells can be given uncrossmatched.
Coombs (Antiglobulin) Tests
The antihuman globulin (Coombs) test detects IgG or complement coating red cells — these incomplete IgG antibodies do not agglutinate cells directly, so Coombs reagent (anti-IgG/anti-C3) bridges them to cause visible agglutination.
| Feature | Direct Coombs (DCT/DAT) | Indirect Coombs (ICT/IAT) |
|---|---|---|
| Detects | Antibody already on the patient's RBCs (in vivo coating) | Free antibody in serum (in vitro) |
| Method | Add Coombs reagent directly to washed patient RBCs | Patient serum + reagent RBCs → incubate → wash → add Coombs reagent |
| Uses | HDN, autoimmune haemolytic anaemia (AIHA), delayed/drug-induced haemolytic transfusion reaction | Antibody screening, crossmatching, antenatal anti-D titre, weak D detection |
Memory hook — "DIRECT = D for Disease already on cells; INDIRECT = serum screening before transfusion."
High-yield: Antenatal screening for maternal anti-D and the crossmatch both use the indirect Coombs test. A baby with HDN has a positive direct Coombs test (maternal IgG coating fetal cells).
Transfusion Reactions — Mechanisms
Acute Haemolytic Transfusion Reaction (AHTR)
- Cause: ABO incompatibility, almost always a clerical/identification error — wrong unit to wrong patient.
- Mechanism: Recipient's IgM anti-A/anti-B binds donor cells → complement activation → intravascular haemolysis.
- Onset: Within minutes to first hour.
- Features: Fever, chills, loin/back pain, hypotension, haemoglobinuria, oliguria; DIC and acute kidney injury in severe cases. In an anaesthetised patient, diffuse oozing (DIC) and hypotension may be the only clues.
- Management: STOP transfusion immediately → maintain IV line with normal saline → support BP and maintain urine output (forced diuresis with fluids ± furosemide) → treat DIC; recheck patient and unit identity; send samples for DCT, repeat crossmatch, free haemoglobin.
High-yield: The commonest cause of a fatal acute haemolytic reaction is an ABO mismatch from a bedside clerical error, not a laboratory antibody-detection failure. First step in management is always stop the transfusion.
Delayed Haemolytic Transfusion Reaction (DHTR)
- Cause: Anamnestic response — re-exposure to a non-ABO antigen (e.g. Kidd/Jka, Rh, Kell, Duffy) to which the patient was previously sensitised; antibody titre was too low to detect at crossmatch.
- Mechanism: IgG-coated donor cells removed by spleen → extravascular haemolysis.
- Onset: 3–14 days post-transfusion.
- Features: Falling haemoglobin, jaundice, mild fever, positive direct Coombs; usually mild.
- The Kidd (Jk) system is classically implicated because its antibodies disappear quickly from serum, escaping detection.
Other reactions (quick recall)
| Reaction | Mechanism / cause | Key clue |
|---|---|---|
| Febrile non-haemolytic | Recipient antibodies vs donor WBC; cytokines | Commonest reaction; prevent with leucodepletion |
| Allergic / urticarial | IgE vs donor plasma proteins | Itching, hives; antihistamine |
| Anaphylaxis | IgA-deficient recipient with anti-IgA | Give washed cells / IgA-deficient units |
| TRALI | Donor anti-leucocyte antibodies → lung injury | Acute dyspnoea/hypoxia within 6 h; commonest cause of transfusion-related death |
| TACO | Volume overload | Hypertension, raised JVP, pulmonary oedema |
| Bacterial sepsis | Contaminated unit (esp. platelets, stored at room temp) | High fever, shock |
High-yield: TRALI (anti-leucocyte antibodies, hypoxia, normal JVP, treat supportively) vs TACO (circulatory overload, raised JVP/BP, treat with diuretics) is a heavily tested differentiation. TRALI is now the leading cause of transfusion-related mortality.
Haemolytic Disease of the Newborn (HDN)
HDN (erythroblastosis fetalis) results from maternal IgG alloantibodies crossing the placenta and destroying fetal red cells.
Pathophysiology flow (Rh HDN):
Rh-negative mother carries Rh-positive fetus → at delivery (or feto-maternal haemorrhage) fetal D-positive cells enter maternal circulation → mother forms IgG anti-D → first pregnancy usually spared (sensitisation occurs at delivery) → in a subsequent Rh-positive pregnancy, maternal IgG anti-D crosses the placenta → fetal RBC destruction → anaemia, extramedullary haematopoiesis, hydrops fetalis, kernicterus.
| Rh HDN | ABO HDN | |
|---|---|---|
| Setting | Rh-ve mother, Rh+ve fetus | Group O mother, A or B fetus |
| Affected pregnancy | Usually 2nd onward | Can affect the first |
| Severity | Severe (hydrops, kernicterus) | Usually mild |
| Antibody | IgG anti-D | IgG anti-A,B (group O has IgG type) |
| Direct Coombs | Strongly positive | Weakly/variably positive |
High-yield: ABO incompatibility can affect the first pregnancy (because anti-A,B IgG pre-exists in group O mothers) but is usually mild; Rh HDN spares the first pregnancy but is severe in later ones. Only group O mothers cause significant ABO HDN, since only they have IgG (not just IgM) anti-A,B.
Prevention & Management
- Anti-D immunoglobulin (RhIG) given to Rh-negative mothers at 28 weeks and within 72 hours of delivery of an Rh-positive baby (also after abortion, amniocentesis, ectopic, antepartum haemorrhage). It works by clearing fetal D-positive cells before maternal sensitisation (passive–active immunity blockade).
- Kleihauer–Betke test estimates the volume of feto-maternal haemorrhage to calculate the RhIG dose.
- Fetal anaemia monitored by MCA peak systolic velocity on Doppler; severe cases need intrauterine transfusion.
- Neonate: phototherapy, exchange transfusion for severe hyperbilirubinaemia to prevent kernicterus.
High-yield: Anti-D prophylaxis is the classic example of passive immunisation preventing active immunisation. It is useless once the mother is already sensitised (positive antibody screen).
Universal Donor & Recipient and Blood Product Selection
- Universal donor of packed red cells: O-negative (no A, B or D antigens to be attacked). Use uncrossmatched O-negative in life-threatening emergencies.
- Universal recipient: AB-positive (no anti-A, anti-B in plasma).
- For plasma (FFP) the logic reverses: AB plasma is the universal donor (no antibodies), and O is the universal plasma recipient.
| Product | Universal donor | Universal recipient |
|---|---|---|
| Packed red cells | O negative | AB positive |
| Plasma / FFP | AB | O |
High-yield: For red cells, O-negative is universal donor; for plasma, AB is universal donor — opposite directions because the relevant component (antigen vs antibody) differs. This reversal is a favourite MCQ.
Blood components — quick selection
- Packed RBCs — symptomatic anaemia; raise Hb ~1 g/dL per unit.
- FFP — coagulation factor replacement (e.g. liver disease, warfarin reversal when PCC unavailable, DIC).
- Cryoprecipitate — rich in fibrinogen, factor VIII, vWF, factor XIII, fibronectin; used in hypofibrinogenaemia, DIC.
- Platelets — thrombocytopenia with bleeding; stored at 20–24°C (room temp), hence high sepsis risk.
Complications of Transfusion (overview)
- Immunological: AHTR, DHTR, febrile/allergic reactions, anaphylaxis, TRALI, post-transfusion purpura, graft-versus-host disease (give irradiated cells to prevent).
- Infectious: Hepatitis B/C, HIV, malaria, syphilis, CMV, bacterial sepsis.
- Massive transfusion: hypocalcaemia (citrate toxicity), hyperkalaemia, hypothermia, dilutional coagulopathy and thrombocytopenia, metabolic alkalosis.
- Iron overload (haemosiderosis): chronic transfusion (e.g. thalassaemia) → treat with iron chelators.
High-yield: Citrate anticoagulant in stored blood chelates calcium → hypocalcaemia during massive transfusion; stored blood also leaks potassium → hyperkalaemia. Both are classic massive-transfusion MCQ answers.
Key Differentials
- Intravascular vs extravascular haemolysis: intravascular (ABO, IgM, complement) → haemoglobinaemia/uria, low haptoglobin; extravascular (Rh, IgG, splenic) → jaundice, splenomegaly, positive DCT.
- TRALI vs TACO: see table above (hypoxia with normal JVP vs overload with raised JVP).
- AHTR vs febrile non-haemolytic reaction: haemolytic shows haemoglobinuria, DIC, renal failure; febrile shows isolated fever with no haemolysis.
- Rh vs ABO HDN: severity and which pregnancy is affected.
Recently asked / exam angle
- Bombay phenotype — types as O but incompatible with O blood; has anti-H; needs autologous/Bombay donor. Recurrent NEET PG favourite.
- Indirect Coombs test uses: antibody screening, crossmatch, antenatal anti-D titre, weak-D typing. Direct Coombs: HDN, AIHA, transfusion reaction work-up.
- Universal donor reversal for red cells (O-ve) vs plasma (AB).
- First step in acute transfusion reaction = STOP the transfusion.
- Sugar added by A enzyme = N-acetylgalactosamine; by B enzyme = galactose. O has most H antigen.
- TRALI = commonest cause of transfusion-related death, mechanism = donor anti-leucocyte antibodies, onset within 6 hours.
- Anti-D timing: 28 weeks + within 72 h of delivery; given to Rh-negative unsensitised mothers only.
- Kidd (Jka) antibodies → classic cause of delayed haemolytic transfusion reaction.
- Citrate toxicity → hypocalcaemia in massive transfusion.
Rapid revision
- ABO gene on chromosome 9; Rh genes (RHD/RHCE) on chromosome 1.
- ABO antibodies = IgM, natural, intravascular haemolysis; Rh anti-D = IgG, immune, extravascular, crosses placenta.
- Landsteiner's law: serum has antibody to the antigen the RBC lacks; antibodies appear by 3–6 months.
- Group O = most H antigen; AB = least; Bombay (hh) = no H, has anti-H, needs Bombay blood.
- A enzyme adds N-acetylgalactosamine; B enzyme adds galactose.
- Major crossmatch = donor cells + recipient serum (most important).
- Direct Coombs = antibody on patient's cells (HDN, AIHA); Indirect Coombs = serum antibody (screen, crossmatch, antenatal).
- AHTR = ABO clerical error, intravascular, DIC + AKI → stop transfusion first.
- DHTR = anamnestic, 3–14 days, Kidd antigen, positive DCT, extravascular.
- TRALI (hypoxia, <6 h, leading transfusion death) vs TACO (volume overload, raised JVP).
- Rh HDN spares first pregnancy/severe; ABO HDN can hit first pregnancy/mild (group O mother).
- Anti-D at 28 weeks + 72 h post-delivery; red-cell universal donor = O-ve, plasma universal donor = AB.