Subtitle: Immunohaematology test-your-knowledge series 8, rebuilt as a guide to blood group serology: ABO and H, Rh and weak D, the clinically important antigen systems, the antibody screen and identification, the direct antiglobulin test, crossmatching, haemolytic disease of the fetus and newborn, transfusion reactions and 10 MCQs.
Author: PathologyMCQ Editorial Board
Category: Transfusion Medicine
Last Updated: October 2026
Read Time: 8 minutes
At-a-Glance
- ABO: naturally occurring IgM anti-A/B, immediate intravascular haemolysis; Bombay lacks H and has anti-H.
- Rh D is the most immunogenic antigen, then K; both cause HDFN.
- Kidd antibodies cause delayed haemolytic reactions; Duffy null protects against P. vivax; Lewis antigens are adsorbed from plasma.
- Antibody screen: group O reagent cells by IAT; identification by panel; DAT for in vivo coating.
- Transfusion reactions: ABO error (acute), Kidd (delayed), FNHTR (cytokines, leukoreduction), TRALI (donor antibodies), TACO (overload), anaphylaxis (IgA deficiency).
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ABO and H
- H antigen is made by FUT1 (red cells) and FUT2 (secretions); A and B transferases add N-acetylgalactosamine or galactose to H. Group O has the most H; A1 has the least.
- Bombay (Oh): no H, so no A or B; serum contains anti-A, anti-B and potent anti-H; compatible only with Oh donors. Para-Bombay: FUT1 null but FUT2 active, so small amounts of A/B are adsorbed.
- Subgroups: A1 (80%) and A2 (20%); A2 and A2B individuals may make anti-A1, usually clinically insignificant. Weak subgroups (A3, Ax) give mixed-field or weak reactions.
- Antibodies: naturally occurring, mainly IgM, appear by 3–6 months, titres fall in the elderly and in immunodeficiency; group O people also have IgG anti-A,B that can cause mild HDFN.
- Discrepancies: forward (cell) versus reverse (serum) mismatch from age, immunodeficiency, transplant, acquired B (bacterial deacetylase in colon cancer), rouleaux, cold agglutinins, subgroups, transfusion and chimerism.
The Rh System
- Five principal antigens: D, C, c, E, e; RHD and RHCE genes on chromosome 1. D-negative in Europeans is usually RHD deletion; in Africans often an RHD pseudogene.
- Common haplotypes: DCe (R1), DcE (R2), dce (r), Dce (R0).
- Weak D: fewer D antigens, detected at the IAT phase; types 1, 2 and 3 do not make anti-D and can be managed as D-positive. Partial D: missing epitopes, may make anti-D; manage as D-negative for transfusion and anti-D prophylaxis. RHD genotyping resolves serological weak D.
- Rh antibodies are immune IgG, do not fix complement, cause extravascular haemolysis and HDFN; anti-D remains the commonest cause of severe HDFN, followed by anti-K and anti-c.
- Rh null: stomatocytosis and mild haemolysis.
Other Clinically Important Systems
Blood group systems that matter at the bench
| System | Antigens | Antibody characteristics | Clinical significance |
|---|---|---|---|
| Kell | K, k, Kp(a/b), Js(a/b); on a transmembrane endopeptidase | Immune IgG | Second most immunogenic; anti-K HDFN suppresses erythropoiesis (low reticulocytes, low amniotic bilirubin); McLeod phenotype (Kx) with acanthocytes and CGD |
| Duffy | Fy(a), Fy(b) | Immune IgG; destroyed by enzymes (papain) | Fy(a-b-) protects against P. vivax; antibodies cause HTR and HDFN |
| Kidd | Jk(a), Jk(b) | IgG, complement-binding, show dosage, fall quickly below detection | Classic cause of delayed haemolytic transfusion reactions |
| MNS | M, N (glycophorin A); S, s, U (glycophorin B) | Anti-M, anti-N usually cold IgM; anti-S, anti-s, anti-U IgG | Anti-S/s/U cause HTR and HDFN; U-negative in Africans |
| Lewis | Le(a), Le(b); adsorbed from plasma | IgM, cold | Rarely significant; antigens may disappear in pregnancy |
| P1PK and Globoside | P1, P, P(k) | Anti-P1 cold IgM; anti-P (Donath–Landsteiner) biphasic IgG | Paroxysmal cold haemoglobinuria; anti-PP1P(k) causes recurrent miscarriage |
| I/i | I (adult), i (newborn, cord) | Cold IgM autoantibodies | Cold agglutinin disease (anti-I after Mycoplasma, anti-i after EBV) |
| Lutheran, Diego, Colton, Dombrock and others | Various | Variable | Occasional HTR; Di(a) in East Asians and Native Americans |
The Tests: Screen, Panel, DAT, Crossmatch
Pre-transfusion and immunohaematology tests
| Test | Method | Detects | Interpretation |
|---|---|---|---|
| ABO and D grouping | Forward (cells with anti-A, anti-B, anti-D) and reverse (plasma with A1 and B cells) | Antigens and expected antibodies | Must agree; resolve discrepancies before transfusion |
| Antibody screen | Patient plasma versus 2–3 group O reagent cells by IAT (tube, gel or solid phase) at 37 °C | Clinically significant IgG alloantibodies | Positive screen goes to a panel |
| Antibody identification | Panel of 10–16 cells with known phenotypes; rule-out and rule-in; enzyme and other techniques | Specificity | Patient phenotype should be antigen-negative; dosage and enzyme behaviour help |
| Direct antiglobulin test (DAT) | Patient red cells with polyspecific, then anti-IgG and anti-C3d | In vivo coating | Positive in AIHA (IgG warm, C3d cold), HDFN, HTR, drug-induced haemolysis; 1–3% of healthy donors and many hospital patients |
| Elution | Acid or heat elution of antibody from DAT-positive cells | Specificity of coating antibody | Identifies the alloantibody in HTR and HDFN |
| Crossmatch | Immediate spin (ABO check), electronic (software) or full IAT crossmatch | Incompatibility | IAT crossmatch when alloantibodies present or previously known |
| Antigen typing | Serology or genotyping | Patient and donor antigens | Extended matching for sickle cell disease (Rh, K) and thalassaemia |
| Titration | Serial dilutions versus antigen-positive cells | Antibody strength | Monitoring anti-D and anti-K in pregnancy; ABO-incompatible transplant |
Haemolytic Disease of the Fetus and Newborn
- Maternal IgG crosses the placenta from about 12 weeks; fetal red cells coated with antibody are destroyed in the fetal spleen, causing anaemia, hydrops and, after birth, hyperbilirubinaemia and kernicterus.
- Anti-D: prevented by routine antenatal anti-D prophylaxis (28 weeks, and after sensitising events), and post-natal anti-D dosed by the Kleihauer or flow cytometric fetomaternal haemorrhage estimate.
- Anti-K: anaemia out of proportion to bilirubin because erythroid precursors are targeted.
- ABO HDFN: group O mother, A or B infant; mild, positive or weak DAT, spherocytes, rarely needs exchange.
- Monitoring: antibody titres and quantification (anti-D in IU/mL), middle cerebral artery peak systolic velocity, intrauterine transfusion with group O, D-negative, K-negative, CMV-safe, irradiated blood.
Transfusion Reactions
Transfusion reactions compared
| Reaction | Timing | Mechanism | Features | Laboratory | Prevention |
|---|---|---|---|---|---|
| Acute haemolytic | Minutes | ABO incompatibility (clerical error), IgM, complement | Fever, flank pain, hypotension, haemoglobinuria, DIC, renal failure | Positive DAT, free plasma Hb, repeat group on pre- and post-samples | Bedside identification checks |
| Delayed haemolytic | 3–14 days | Anamnestic IgG alloantibody (Kidd, Duffy, Rh, K) | Falling Hb, jaundice, spherocytes | Positive DAT, new antibody, positive eluate | Antigen-negative units; antibody records |
| Febrile non-haemolytic | Within 4 h | Recipient anti-leukocyte antibodies or stored cytokines | Fever, rigors | Exclude haemolysis and sepsis | Leukoreduction |
| Allergic / anaphylactic | Minutes | Plasma proteins; IgA deficiency with anti-IgA | Urticaria to shock | IgA level, anti-IgA | Washed cells, IgA-deficient plasma |
| TRALI | Within 6 h | Donor anti-HLA or anti-HNA antibodies, or lipid primers | Hypoxia, bilateral infiltrates, fever, hypotension | Donor antibody testing | Male or screened plasma donors |
| TACO | Within 12 h | Volume overload | Dyspnoea, hypertension, raised JVP, raised BNP | Chest X-ray, BNP | Slow transfusion, diuretics in at-risk patients |
| Bacterial contamination | During or soon after | Platelets stored at 22 °C | High fever, shock | Culture component and patient | Diversion pouch, bacterial screening |
| TA-GVHD | 1–6 weeks | Donor T cells engraft in immunocompromised or HLA-haploidentical recipients | Rash, pancytopenia, hepatitis, almost always fatal | Chimerism | Irradiation of cellular components |
| Post-transfusion purpura | 5–12 days | Anti-HPA-1a in HPA-1a-negative recipient | Severe thrombocytopenia | HPA antibodies | IVIG; HPA-matched platelets |
| Iron overload | Chronic | Each unit 200–250 mg iron | Organ damage after 20 units | Ferritin, MRI | Chelation |
Test Yourself: 10 MCQs
Immunohaematology: 10 MCQs
ABO and H, Rh, the antibody screen, DAT, crossmatch, blood group systems and transfusion reactions, with instant explanations.
Score: 0 / 10
More practice: the question banks →
Exam Pearls
- Bombay: no H, anti-H, only Oh donors.
- D then K are the most immunogenic; partial D can make anti-D, weak D types 1–3 do not.
- Kidd = delayed haemolysis; Duffy null = vivax resistance; Lewis = adsorbed, insignificant.
- Positive DAT: IgG warm AIHA, C3d cold; eluate finds the specificity.
- TRALI = donor antibodies, no overload; TACO = overload, BNP up.
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References
- Daniels G. Human Blood Groups. 3rd ed. Wiley-Blackwell; 2013.
- AABB. Technical Manual. 21st ed. AABB; 2023.
- Bain BJ, Bates I, Laffan MA, eds. Dacie and Lewis Practical Haematology. 12th ed. Elsevier; 2017.
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