3.2 Immune-Mediated Hemolytic Anemias & Paroxysmal Nocturnal Hemoglobinuria

Immune-Mediated Hemolytic Anemias & Paroxysmal Nocturnal Hemoglobinuria

Immune-mediated hemolytic anemias (AIHA) occur when antibodies, complement proteins, or both bind to erythrocyte surface antigens, accelerating red blood cell destruction through extravascular or intravascular pathways. Paroxysmal Nocturnal Hemoglobinuria (PNH), while non-immune in etiology, shares the clinical hallmark of complement-mediated intravascular destruction resulting from an acquired clonal deficiency in complement regulatory surface proteins.

                  IMMUNE & COMPLEMENT-MEDIATED LYSIS
                                  |
       +--------------------------+--------------------------+
       |                                                     |
AUTOIMMUNE HEMOLYTIC ANEMIAS                           CLONAL STEM CELL
       |                                                     |
 +-----+-----+-----+                                   PNH (PIGA Mutation)
 |           |     |                                   - Deficient GPI Anchor
WARM        COLD  PCH                                  - Loss of CD55 & CD59
(IgG, 37°C) (IgM) (Biphasic IgG)                       - Intravascular Lysis

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1. Direct Antiglobulin Test (DAT) Principles and Diagnostic Strategy

The Direct Antiglobulin Test (DAT / Direct Coombs Test) detects in vivo sensitization of patient red blood cells by immunoglobulins (IgG) and/or complement components (primarily C3d).

                         DAT TESTING ALGORITHM
                                   |
               [ Patient EDTA Whole Blood / Washed RBCs ]
                                   |
                     [ Polyspecific AHG Reagent ]
                     (Anti-Human IgG + Anti-C3d)
                                   |
               +-------------------+-------------------+
               |                                       |
          Negative DAT                            Positive DAT
               |                                       |
      (Non-Immune Hemolysis:                 [ Monospecific AHG Testing ]
       HS, HE, MAHA, PNH, etc.)               +--------+--------+
                                              |                 |
                                         Anti-IgG           Anti-C3d
                                              |                 |
                                       - WAIHA (80%)      - CAD (IgM)
                                       - Drug-Adsorption  - PCH (DL)
                                       - Alloantibodies   - Immune Complex Drug

Stepwise Testing Workflow

1. Polyspecific AHG Screen: Washed patient RBCs are tested with polyspecific Anti-Human Globulin containing antibodies against human IgG and the complement fragment C3d.
2. Monospecific AHG Differentiation: If the polyspecific screen is positive, testing reflexes to monospecific anti-IgG and monospecific anti-C3d:
   • IgG positive, C3d negative: Typical for warm autoimmune hemolytic anemia (~20–40% of WAIHA cases) and drug-adsorption (hapten) mechanisms.
   • IgG positive, C3d positive: Seen in ~50–60% of WAIHA cases.
   • IgG negative, C3d positive: Classic for Cold Agglutinin Disease (CAD), Paroxysmal Cold Hemoglobinuria (PCH), immune-complex drug mechanisms, and certain delayed hemolytic transfusion reactions.
3. Indirect Antiglobulin Test (IAT): Evaluates in vitro sensitization by incubating normal reagent RBCs with patient serum/plasma to identify free, unadsorbed circulating autoantibodies or alloantibodies.

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2. Warm Autoimmune Hemolytic Anemia (WAIHA)

Immunological Mechanism & Pathogenesis

Warm Autoimmune Hemolytic Anemia accounts for 70–80% of all autoimmune hemolytic anemia cases. It is mediated by warm-reactive IgG autoantibodies (most commonly IgG1 and IgG3 subclasses) that react optimally at $37^\circ\text{C}$. The autoantibodies are usually directed against ubiquitous, high-incidence antigens of the Rh complex (e.g., pan-anti-Rh, anti-e, or core Rh structural proteins).

Warm-Reactive IgG Autoantibody Binds RBC at 37°C
  --> Splenic Red Pulp Transit (Cords of Billroth)
  --> Macrophage Fc-gamma Receptors (Fc-gamma-RI, Fc-gamma-RIIa) Bind IgG Fc Region
  --> Partial Phagocytosis / Membrane Stripping
  --> Loss of Surface Area with Preserved Volume
  --> Microspherocyte Formation
  --> Rigid Spherocyte Trapped & Destroyed (Extravascular Hemolysis)

Clinical Associations

• Idiopathic / Primary: ~50% of cases.
• Secondary WAIHA: Strongly associated with Chronic Lymphocytic Leukemia (CLL) and low-grade B-cell non-Hodgkin lymphomas, Systemic Lupus Erythematosus (SLE), Evans syndrome (WAIHA + Immune Thrombocytopenia), and Common Variable Immunodeficiency (CVID).

Laboratory & Smear Features

• Smear: Prominent microspherocytes, polychromasia, nucleated RBCs, and occasional erythrophagocytosis by monocytes.
• DAT Result: Positive for IgG alone ($20\text{--}40%$) or IgG + C3d ($50\text{--}60%$).
• Differentiation from Hereditary Spherocytosis: Both display spherocytes, high RDW, and elevated MCHC. WAIHA is differentiated by a positive DAT, an acquired/acute clinical onset, absence of a family history, and a positive eluate containing an IgG autoantibody.

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3. Cold Agglutinin Disease (CAD) & Cold Autoantibody Syndromes

Immunological Mechanism

Cold Agglutinin Disease accounts for 15–25% of AIHA cases. It is mediated by cold-reactive IgM autoantibodies (occasionally IgA) that bind maximally at $4^\circ\text{C}\text{ to }18^\circ\text{C}$. In pathogenic states, the thermal amplitude extends toward core body temperature ($>30^\circ\text{C}$).

Peripheral Capillaries (28-32°C): IgM Pentamer Binds RBC Antigen (Anti-I / Anti-i)
  --> Classical Complement Cascade Activated: C1q --> C4 --> C2 --> C3b
  --> Central Circulation (37°C): IgM Dissociates from RBC Surface
  --> C3b Remains Covalently Bound to RBC
  --> Factor I Cleaves C3b into Stable C3d
  --> Extravascular Clearance in Liver Kupffer Cells (via CR3 Receptors)

• Antigen Specificity:
  • Anti-I Specificity: Most common in idiopathic CAD (monoclonal IgM$\kappa$ lymphoproliferative clones) and post-Mycoplasma pneumoniae infection (polyclonal IgM).
  • Anti-i Specificity: Classically associated with infectious mononucleosis (Epstein-Barr Virus / EBV) and Cytomegalovirus (CMV). Fetal/neonatal erythrocytes express abundant linear i-antigen, which branches into complex I-antigen during the first two years of life.

Laboratory Analyzer Interference & Spurious Results

When whole blood from a patient with high-titer cold agglutinins cools to room temperature in an EDTA tube, erythrocytes spontaneously agglutinate into macroscopic clusters. This causes massive, characteristic automated analyzer errors:

                     COLD AGGLUTININ CBC ARTIFACTS

Agglutinated RBC Clusters Passed Through Aperture / Optical Flow Cell
  --> Falsely Decreased RBC Count (Multiple RBCs counted as 1 event)
  --> Falsely Elevated MCV (>130 fL, mega-clusters)
  --> Falsely Decreased Hematocrit (Hct = RBC x MCV / 10)
  --> Falsely Elevated MCHC (Frequently 45 - 65 g/dL; Physio Max ~37 g/dL)
  --> Rule of Three Violations: (Hb x 3 =/= Hct)

Corrective Action Protocol:
1. Place the EDTA sample in a 37°C water bath or incubator for 15–30 minutes.
2. Re-run immediately on the automated analyzer.
3. Result: Complete dispersion of autoagglutinated clusters, normalizing RBC, MCV, and MCHC.

• DAT Profile: Positive for C3d ONLY; negative for IgG. Because IgM dissociates during the $37^\circ\text{C}$ saline washing phase, no immunoglobulin remains on the membrane.

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4. Paroxysmal Cold Hemoglobinuria (PCH)

Donath-Landsteiner Biphasic Antibody

Paroxysmal Cold Hemoglobinuria is a distinct autoimmune hemolytic anemia caused by the Donath-Landsteiner (D-L) antibody—a biphasic IgG autoantibody directed against the high-incidence P antigen ($P/P_1P^k$) on red blood cells.

                         BIPHASIC MECHANISM OF PCH

1. Peripheral Capillaries (4°C - Cold Phase):
   IgG Donath-Landsteiner Antibody Binds to P Antigen
   Early Complement Components (C1q, C4, C2) are Fixed to RBC
                              |
                              v
2. Central Warm Circulation (37°C - Warm Phase):
   Full Complement Activation: C3 Convertase --> C5b-9 (MAC Assembly)
   Rapid Membrane Perforation
                              |
                              v
3. Massive INTRAVASCULAR Hemolysis:
   Hemoglobinemia, Severe Hemoglobinuria ("Port-Wine Urine"), Rigors, Chills

• Epidemiology: Historically associated with tertiary syphilis; today, it is predominantly observed as an acute, transient, self-limiting post-viral hemolytic syndrome in children following upper respiratory infections, measles, mumps, varicella, influenza, or CMV.
• Donath-Landsteiner Confirmatory Test: Patient serum and normal group O P-antigen-positive red cells are incubated under two conditions:
  • Test (Biphasic): Incubated at $4^\circ\text{C}$ for 30 minutes, then warmed to $37^\circ\text{C}$ for 60 minutes $\to$ Gross Hemolysis.
  • Control 1 (Cold Only): Maintained at $4^\circ\text{C}$ throughout $\to$ No Hemolysis.
  • Control 2 (Warm Only): Maintained at $37^\circ\text{C}$ throughout $\to$ No Hemolysis.
• DAT Result: Weakly positive for C3d only during or immediately following the acute attack.

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5. Drug-Induced Immune Hemolytic Anemia (DIIHA)

Drug-induced immune hemolysis occurs via three classical immunological mechanisms:

DRUG MECHANISMS:
1. Drug-Adsorption (Hapten)    : Drug binds RBC --> IgG anti-drug binds --> Spleen extravascular
2. Immune Complex (Bystander) : Drug + Ab in plasma --> Adsorbs to RBC --> Fixes MAC --> Intravascular
3. Autoantibody Induction     : Drug alters T-cell tolerance --> True anti-RBC IgG --> WAIHA-like

| Mechanism | Prototype Drugs | Immunological Mechanism | DAT Result | Clinical Manifestation | | :--- | :--- | :--- | :--- | :--- | :--- | | Drug-Adsorption (Hapten) | High-dose IV Penicillin, Ampicillin, Cephalosporins | Drug coats RBC surface covalently; high-titer IgG anti-drug antibody binds to drug on RBC | IgG Positive (C3d negative) | Subacute extravascular hemolysis in spleen; resolves upon drug cessation | | Immune Complex ("Innocent Bystander") | Quinidine, Ceftriaxone, Rifampin, Stibophen | Drug-antibody immune complex forms in plasma, non-specifically adsorbs to RBC, fixes complement, and dissociates | C3d Positive ONLY (IgG negative) | Hyperacute, severe intravascular hemolysis, hemoglobinuria, acute renal failure | | Autoantibody Induction | $\alpha$-Methyldopa, Fludarabine, Procainamide | Drug alters suppressor T-cell regulation, inducing true autoantibodies against Rh antigens | IgG Positive (indistinguishable from WAIHA) | Extravascular hemolysis; antibody reacts in vitro even in total absence of drug | | Non-Immunologic Protein Adsorption (NIPA) | Cephalosporins, Cefotetan, $\beta$-lactamase inhibitors | Drug modifies RBC membrane, causing non-immunological sticky adsorption of plasma proteins (IgG, albumin, fibrinogen) | IgG Positive / Poly Positive | No hemolysis; positive DAT is an in vitro laboratory artifact |

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6. Paroxysmal Nocturnal Hemoglobinuria (PNH)

Molecular Pathogenesis & The PIGA Mutation

Paroxysmal Nocturnal Hemoglobinuria is an acquired, non-malignant, clonal hematopoietic stem cell disorder caused by somatic loss-of-function mutations in the PIGA (phosphatidylinositol glycan anchor biosynthesis class A) gene located on the X chromosome (Xp22.2).

Somatic PIGA Mutation in Multipotent Hematopoietic Stem Cell
  --> Failure to Synthesize Glycosylphosphatidylinositol (GPI) Anchor Core
  --> Complete Absence of GPI-Anchored Membrane Protective Proteins
  --> Primary Loss of CD55 (DAF) & CD59 (MIRL / Protectin)
  --> Loss of Complement Regulation on Erythrocytes & Platelets
  --> Chronic, Uncontrolled Complement-Mediated INTRAVASCULAR Hemolysis

       KEY GPI-ANCHORED REGULATORY PROTEINS DEFICIENT IN PNH

1. CD55 (Decay-Accelerating Factor / DAF):
   - Accelerates decay and disassembly of C3 and C5 convertases (C3bBb and C4b2a)
   - Prevents amplification of complement activation on cell surface

2. CD59 (Membrane Inhibitor of Reactive Lysis / MIRL / Protectin):
   - Binds to C8 and C9 during Terminal Complement Assembly
   - Directly prevents incorporation of C9 into C5b-8
   - BLOCKS FORMATION OF MEMBRANE ATTACK COMPLEX (MAC / C5b-9 pore)
   - LOSS OF CD59 IS THE PRIMARY CAUSE OF INTRAVASCULAR LYSIS IN PNH

Clinical Triad and Pathophysiology

1. Complement-Mediated Intravascular Hemolysis: Intravascular lysis releases free oxyhemoglobin into plasma. Hemoglobinuria is classically concentrated during sleep ("nocturnal") because mild hypoventilation produces slight respiratory acidosis, enhancing alternative complement pathway activation. Chronic hemosiderinuria causes severe renal tubular hemosiderosis and progressive iron deficiency.
2. High-Risk Venous Thrombosis: The leading cause of mortality in PNH ($>40%$ of patients). Thromboses occur at unusual anatomical sites: hepatic veins (Budd-Chiari syndrome), mesenteric veins, splenic veins, portal veins, cerebral sagittal sinus, and dermal veins. Driven by free hemoglobin scavenging nitric oxide (NO), causing smooth muscle dystonia, systemic vasoconstriction, esophageal spasms, erectile dysfunction, and platelet hyperactivity.
3. Bone Marrow Failure: Strong pathogenic overlap with Aplastic Anemia (AA) and low-risk Myelodysplastic Syndromes (MDS). In aplastic anemia, auto-reactive T-cells target GPI-expressing stem cells, allowing GPI-negative (PNH) clones to survive via immune escape.

Diagnostic Flow Cytometry (The Gold Standard)

Historical tests like the Ham test (Acidified Serum Lysis) and the Sucrose Hemolysis Test are completely obsolete due to poor sensitivity and specificity.

• High-Sensitivity Flow Cytometry: Evaluates the loss of GPI-anchored markers on red blood cells and white blood cells (neutrophils and monocytes):
  • FLAER (Fluorescent Aerolysin): A fluorochrome-conjugated inactive bacterial aerolysin variant that binds directly and specifically to the glycan core of intact GPI anchors. Used to analyze granulocytes and monocytes with extreme sensitivity (detecting clones $<0.01%$).
  • Erythrocyte Gating: CD59 and CD55 expression classifies RBCs into:
    • Type I Cells: Normal GPI expression (CD59/CD55 fully present).
    • Type II Cells: Partial deficiency (intermediate complement sensitivity).
    • Type III Cells: Complete deficiency (extreme complement sensitivity; $\approx 10\text{-fold}$ more sensitive to lysis).
  • Leukocyte Confirmation: Because transfusions dilute Type III RBCs, quantifying GPI deficiency on granulocytes (FLAER + CD24 / CD157) and monocytes (FLAER + CD14 / CD157) is required for definitive diagnostic clone size assessment.

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7. Comparative Diagnostic Profile