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Autoimmune Hemolytic Anemia (AIHA) — Complete Pathology for NEET PG 2026

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1. Definition & Overview

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2. Classification (Most Exam-Important)

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3. Warm AIHA — Detailed Pathology

Etiology

• Primary (Idiopathic): ~50% of cases
• Secondary:
  • Lymphoproliferative: CLL (most common), NHL, Hodgkin lymphoma
  • Autoimmune: SLE (most important), RA, IBD
  • Infections: EBV, HIV
  • Drugs: methyldopa, fludarabine
  • Solid tumors (rare)

Antibody Characteristics

• IgG (subtypes IgG1, IgG3 are most hemolytic)
• Non-complement fixing (usually) — does NOT activate complement fully
• Directed against Rh antigens most commonly (pan-agglutinin — reacts with all Rh+ cells)
• React best at 37°C

Mechanism of Hemolysis (EXTRAVASCULAR)

1. IgG antibodies coat the RBC surface
2. Coated RBCs travel to spleen
3. Splenic macrophages recognize IgG via Fc receptors (FcγRIII)
4. Macrophages partially phagocytose the RBC membrane → microspherocytes form
5. Microspherocytes are rigid, trapped in splenic sinusoids → destroyed
6. Result: extravascular hemolysis → splenomegaly
7. If IgG1/IgG3 + complement activation → C3b on RBC → Kupffer cells in liver (via CR1) also destroy RBCs

Key point for MCQs: Warm AIHA → IgG → Spleen → Extravascular hemolysis → Microspherocytes

Lab Findings

• Anemia (normocytic/macrocytic if compensated)
• Reticulocytosis (usually — but can have reticulocytopenia in severe cases)
• Peripheral smear: Microspherocytes (most characteristic), polychromasia
• Raised LDH, raised indirect bilirubin, raised urobilinogen
• Low/absent haptoglobin
• Direct Coombs test (DAT) POSITIVE for IgG ± C3
• Indirect Coombs test (IAT): may be positive (free antibody in serum)

Direct Antiglobulin Test (DAT / Direct Coombs Test)

• Developed by R.R.A. Coombs in 1945
• Uses broad-spectrum reagent detecting both Ig AND complement (C3 fragments) on RBC surface
• Positive if ≥400 molecules of Ig/C3 per RBC
• Practically diagnostic of AIHA when positive (barring recent transfusion)
• In Warm AIHA: IgG positive ± C3 positive
• Advanced techniques (flow cytometry) detect even lower coating levels

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4. Cold Agglutinin Disease (CAD)

Etiology

• Primary (Idiopathic): Chronic CAD in elderly
• Secondary:
  • Mycoplasma pneumoniae → anti-I antibody (IgM)
  • Infectious mononucleosis (EBV) → anti-i antibody (IgM)
  • Lymphoproliferative disease (Waldenström macroglobulinemia)

Antibody Characteristics

• IgM (pentameric — very efficient at complement fixation)
• Directed against I antigen (Mycoplasma, primary CAD) or i antigen (EBV)
• Reacts at 0–4°C, thermal amplitude up to 30°C (pathological if >30°C)

Mechanism

1. In peripheral cold areas (acral regions): IgM binds I/i antigen → activates complement (C1→C4b2a→C3b deposited)
2. IgM detaches as blood warms (it's reversible at 37°C)
3. RBCs coated with C3b → liver (Kupffer cells) via CR1 → extravascular hemolysis
4. If complement cascade goes to completion → MAC (C5b-9) → intravascular hemolysis + hemoglobinuria

DAT Result

• C3d POSITIVE, IgG NEGATIVE (IgM washes off during test; only C3 fragments remain)

Clinical Features

• Acrocyanosis (fingers, toes, ears, nose turn blue/purple in cold)
• Hemolysis worsens in cold weather
• Raynaud phenomenon
• Mild chronic anemia

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5. Paroxysmal Cold Hemoglobinuria (PCH)

Key Facts (HIGH YIELD)

• Rarest type of AIHA
• Classically associated with syphilis (tertiary/congenital); now more common post-viral infections in children
• Antibody: Donath-Landsteiner (DL) antibody — IgG, biphasic

Donath-Landsteiner Antibody (MUST KNOW)

• Directed against P antigen on RBCs
• Biphasic hemolysis: antibody binds in cold, hemolysis occurs on rewarming
• DAT: C3 positive, IgG may be weakly positive or negative (IgG detaches)
• Hemoglobinuria after cold exposure (paroxysmal)
• Donath-Landsteiner test: specific diagnostic test

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6. Drug-Induced AIHA

Three Mechanisms (Classic NEET Topic)

Methyldopa: 15–20% develop positive DAT; only 1% develop actual hemolysis. Antibody is directed at Rh antigens.

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7. Peripheral Blood Smear Findings

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8. Summary of DAT Results (Ultra High-Yield Table)

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9. General Lab Features of Hemolytic Anemia (All AIHA Types)

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10. Treatment Overview (for Context)

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11. Exam-Specific One-Liners

• Warm AIHA: IgG, 37°C, extravascular, spleen, microspherocytes, DAT IgG+
• Cold agglutinitis: IgM, cold, complement-mediated, liver, DAT C3+ only, anti-I (Mycoplasma) / anti-i (EBV)
• PCH: IgG biphasic, P antigen, Donath-Landsteiner Ab, syphilis/post-viral, intravascular, DAT C3+
• Coombs test (1945, Coombs) = positive = diagnostic of AIHA
• DAT = detects Ab ON patient RBC; IAT = detects Ab IN patient serum
• Methyldopa: 15–20% positive DAT, only 1% actual hemolysis, anti-Rh
• Quinidine: innocent bystander, C3+, intravascular hemolysis
• Penicillin (high dose): hapten mechanism, IgG+, extravascular
• Microspherocytes in AIHA = partial splenic phagocytosis (NOT osmotic fragility like hereditary spherocytosis, though osmotic fragility IS increased in both)
• Reticulocytopenia in AIHA = parvovirus B19 co-infection or autoantibodies against reticulocytes → aplastic crisis

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12. Associations to Remember for MCQs

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