Acquired Hemolytic

Overview of Acquired [Immune-Mediated] Hemolytic Anemias

Definition: Hemolysis (RBC destruction) that occurs due to the binding of antibodies and/or complement proteins to the surface of RBCs
Hallmark: A positive Direct Antiglobulin Test (DAT), also known as a Coombs test. The DAT detects the presence of antibodies or complement proteins bound to RBCs
Classification:
- Autoimmune Hemolytic Anemia (AIHA): Antibodies are directed against the individual’s own RBC antigens
  - Warm AIHA
  - Cold AIHA (Cold Agglutinin Disease)
  - Paroxysmal Cold Hemoglobinuria (PCH)
- Drug-Induced Hemolytic Anemia: Antibodies are produced as a result of drug exposure
- Alloimmune Hemolytic Anemia: Antibodies are directed against foreign RBC antigens, typically following transfusion or during pregnancy
  - Hemolytic Disease of the Fetus and Newborn (HDFN)
  - Hemolytic Transfusion Reactions

Mechanisms of Immune-Mediated Hemolysis

The Players
- Red Blood Cells (RBCs): Our innocent bystanders in this scenario, carrying antigens on their surface that can be mistakenly targeted by the immune system
- Antibodies: Immunoglobulins (IgG or IgM) produced by B lymphocytes that can bind to specific antigens on the RBC surface
- Complement System: A group of plasma proteins that, when activated, can lead to cell lysis, inflammation, and enhanced phagocytosis
- Mononuclear Phagocyte System (MPS) / Reticuloendothelial System (RES): A network of phagocytic cells (macrophages) located throughout the body (especially in the spleen and liver) that engulf and destroy antibody- and/or complement-coated RBCs
The Process
1. Sensitization (Antibody Binding):
  - The process begins when antibodies bind to antigens on the surface of RBCs. These antigens can be:
    - Self-antigens: In autoimmune hemolytic anemia (AIHA), the antibodies are autoantibodies that mistakenly recognize the individual’s own RBC antigens as foreign
    - Drug-related antigens: In drug-induced hemolytic anemia, the antibodies may be directed against a drug that has adsorbed to the RBC surface or against a drug-antibody complex
    - Foreign antigens: In alloimmune hemolytic anemia, the antibodies are alloantibodies that recognize foreign RBC antigens from a previous transfusion or from the fetus during pregnancy
  - The type of antibody involved (IgG or IgM) and the density of antigen sites on the RBC influence the subsequent steps
2. Activation of the Complement System (Complement-Mediated Hemolysis):
  - This is more common with IgM antibodies, but IgG can also activate complement
  - The classical pathway of complement activation is triggered when C1q binds to the Fc region of IgM or IgG antibodies on the RBC surface
  - This leads to a cascade of events, resulting in the sequential activation of other complement proteins (C1, C4, C2, C3, C5, C6, C7, C8, and C9)
  - Key Steps:
    - C3 Convertase Formation: C4b2a (C3 convertase) cleaves C3 into C3a and C3b
    - C5 Convertase Formation: C4b2a3b (C5 convertase) cleaves C5 into C5a and C5b
    - Membrane Attack Complex (MAC) Assembly: C5b initiates the assembly of the MAC (C5b-C6-C7-C8-C9) on the RBC membrane
  - Outcomes of Complement Activation:
    - Intravascular Hemolysis: The MAC inserts into the RBC membrane, creating pores that disrupt the cell’s osmotic balance, leading to lysis (rupture) within the blood vessels
    - Opsonization: C3b opsonizes (coats) RBCs, making them more recognizable to phagocytes in the MPS (see below)
    - Anaphylatoxins: C3a and C5a are released and act as anaphylatoxins, promoting inflammation and attracting immune cells
3. Extravascular Hemolysis (MPS-Mediated):
  - Even if the complement cascade isn’t fully activated to the point of forming the MAC, antibody- and/or complement-coated RBCs are targeted for destruction by the MPS, primarily in the spleen and liver
  - The Process:
    - Opsonization: RBCs are coated with IgG antibodies and/or complement fragments (primarily C3b)
    - Recognition by Macrophages: Macrophages in the spleen and liver express:
      - Fc receptors that bind to the Fc region of IgG antibodies
      - Complement receptors (e.g., CR1) that bind to C3b
    - Phagocytosis: Macrophages engulf and destroy the antibody- and/or complement-coated RBCs
  - Consequences of Phagocytosis:
    - RBC Destruction: The RBC is broken down within the macrophage
    - Hemoglobin Breakdown: Hemoglobin is metabolized into:
      - Globin: Broken down into amino acids
      - Iron: Recycled or stored as ferritin/hemosiderin
      - Porphyrin: Converted to bilirubin, which is released into the circulation
Key Players in More Detail
- Antibodies (IgG and IgM):
  - IgG:
    - More efficient at opsonization and promoting extravascular hemolysis
    - Can cross the placenta and cause HDFN
    - Warm AIHA is typically IgG-mediated
  - IgM:
    - More efficient at activating the complement cascade, leading to intravascular hemolysis
    - Cannot cross the placenta (due to its large size)
    - Cold AIHA is typically IgM-mediated
- Complement System:
  - A cascade of plasma proteins that, when activated, can lead to:
    - Cell lysis (formation of the MAC)
    - Opsonization (coating cells to enhance phagocytosis)
    - Inflammation (release of anaphylatoxins)
- Mononuclear Phagocyte System (MPS) / Reticuloendothelial System (RES):
  - A network of phagocytic cells (primarily macrophages) that remove debris and participate in immune responses
  - In immune-mediated hemolysis, the MPS is responsible for:
    - Recognizing and engulfing antibody- and/or complement-coated RBCs
    - Breaking down the RBCs and recycling their components

Types of Immune-Mediated Hemolytic Anemias

Autoimmune Hemolytic Anemia (AIHA)
- Definition: Anemia caused by autoantibodies directed against the individual’s own RBC antigens
- Warm Autoimmune Hemolytic Anemia (Warm AIHA):
  - Antibody Type: IgG
  - Temperature Reactivity: Reacts optimally at body temperature (37°C)
  - Mechanism:
    - IgG antibodies bind to RBCs
    - RBCs are opsonized and phagocytized by macrophages in the spleen (extravascular hemolysis)
    - Some complement activation may occur
  - Etiology:
    - Primary (idiopathic): No underlying cause
    - Secondary: Associated with autoimmune disorders (e.g., SLE), lymphoproliferative disorders (e.g., CLL), infections, or drugs
  - Laboratory Findings:
    - DAT: Positive for IgG (and sometimes complement)
    - Peripheral Blood Smear: Spherocytes, polychromasia
    - Increased reticulocyte count
    - Elevated bilirubin and LDH
    - Decreased haptoglobin
- Cold Autoimmune Hemolytic Anemia (Cold AIHA):
  - Antibody Type: IgM.
  - Temperature Reactivity: Reacts optimally at low temperatures (4°C)
  - Mechanism:
    - IgM antibodies bind to RBCs in colder parts of the body (e.g., extremities)
    - This activates the complement cascade, leading to:
      - Intravascular hemolysis (formation of MAC)
      - Opsonization of RBCs with C3b, leading to splenic sequestration (extravascular hemolysis)
    - IgM antibodies may detach from RBCs at warmer temperatures, but complement fragments remain bound, leading to RBC destruction
  - Etiology:
    - Acute Cold AIHA (Cold Agglutinin Disease): Often associated with infections (e.g., Mycoplasma pneumoniae, infectious mononucleosis). The IgM antibodies are often transient and disappear after the infection resolves
    - Chronic Cold AIHA: Associated with lymphoproliferative disorders (e.g., Waldenström macroglobulinemia)
  - Laboratory Findings:
    - DAT: Positive for complement (C3d) only. IgG is usually negative because IgM detaches from RBCs
    - Peripheral Blood Smear: RBC agglutination (clumping of RBCs)
    - Increased reticulocyte count
    - Elevated bilirubin and LDH
    - Decreased haptoglobin
    - Cold Agglutinin Titer: Elevated (measures the concentration of cold-reacting IgM antibodies)
- Paroxysmal Cold Hemoglobinuria (PCH):
  - Antibody Type: IgG antibody called the Donath-Landsteiner antibody
  - Temperature Reactivity: Binds to RBCs at low temperatures and causes complement-mediated hemolysis upon warming
  - Mechanism:
    - The Donath-Landsteiner antibody binds to the P antigen on RBCs at low temperatures
    - Upon warming to body temperature, the antibody activates the complement cascade, leading to intravascular hemolysis
  - Etiology:
    - Historically associated with syphilis
    - Now more commonly associated with viral infections in children
  - Clinical Features:
    - Sudden onset of hemolytic anemia, often after exposure to cold
    - Hemoglobinuria (dark urine)
    - Fever, chills, abdominal pain
  - Laboratory Findings:
    - CBC: Anemia
    - Peripheral Blood Smear: May show spherocytes, polychromasia
    - Reticulocyte Count: Elevated
    - Elevated bilirubin and LDH
    - Decreased haptoglobin
    - DAT: Positive for complement (C3d) only
    - Donath-Landsteiner Test: Positive (confirms the presence of the Donath-Landsteiner antibody)
Drug-Induced Hemolytic Anemia
- Mechanism:
  - Drug Adsorption: Drug binds to the RBC membrane, and antibodies are directed against the drug-RBC complex
  - Immune Complex Formation: Drug binds to antibody in the plasma, and the immune complex binds to RBCs, leading to complement activation
  - Autoantibody Formation: Drug induces the production of autoantibodies against RBCs (similar to warm AIHA)
- Common Drugs:
  - Penicillin and cephalosporins (drug adsorption)
  - Quinidine and quinine (immune complex formation)
  - Methyldopa (autoantibody formation)
- Laboratory Findings:
  - CBC: Anemia
  - Peripheral Blood Smear: Spherocytes, polychromasia
  - Reticulocyte Count: Elevated
  - DAT: Positive (may be positive for IgG, complement, or both)
  - Drug-specific antibody testing (may be available)
- Treatment: Discontinue the offending drug
Alloimmune Hemolytic Anemia
- Hemolytic Disease of the Fetus and Newborn (HDFN):
  - Mechanism:
    - Maternal alloantibodies (usually IgG) cross the placenta and attack fetal RBCs
    - Most commonly caused by Rh incompatibility (anti-D) or ABO incompatibility
    - Rh Incompatibility: Rh-negative mother is exposed to Rh-positive fetal blood during pregnancy or delivery, leading to the production of anti-D antibodies. In subsequent pregnancies, these antibodies cross the placenta and attack Rh-positive fetal RBCs
    - ABO Incompatibility: Mother with blood type O has anti-A and anti-B IgG antibodies that can cross the placenta and attack fetal RBCs (usually milder than Rh incompatibility)
  - Clinical Features:
    - Fetal anemia, hydrops fetalis (severe edema), jaundice after birth
  - Laboratory Findings:
    - Maternal Antibody Screening: Detects the presence of alloantibodies in the mother’s serum
    - DAT on Newborn RBCs: Positive for IgG (and sometimes complement)
    - Bilirubin: Elevated in the newborn
  - Prevention:
    - RhoGAM (Rh immune globulin) is administered to Rh-negative mothers during pregnancy and after delivery to prevent Rh sensitization
  - Treatment:
    - Intrauterine transfusions for severe fetal anemia
    - Exchange transfusions after birth to reduce bilirubin levels
- Hemolytic Transfusion Reactions:
  - Mechanism:
    - Recipient produces alloantibodies against donor RBC antigens (e.g., anti-A, anti-B, anti-Rh antibodies)
    - Transfused RBCs are attacked by recipient antibodies, leading to hemolysis
  - Types:
    - Acute Hemolytic Transfusion Reaction: Occurs within minutes to hours after transfusion. Symptoms include fever, chills, chest pain, back pain, and hemoglobinuria
    - Delayed Hemolytic Transfusion Reaction: Occurs days to weeks after transfusion. Symptoms may be milder
  - Laboratory Findings:
    - DAT: Positive on post-transfusion RBCs
    - Antibody Identification: Detects alloantibodies in the patient’s serum
    - Elevated bilirubin and LDH
    - Decreased haptoglobin
  - Treatment:
    - Stop the transfusion immediately
    - Supportive care (IV fluids, vasopressors)
    - Manage complications (e.g., acute renal failure, DIC)

General Laboratory Findings in Immune-Mediated Hemolytic Anemias

Complete Blood Count (CBC):
- Anemia (low HGB and HCT)
- MCV: May be normocytic or macrocytic (due to reticulocytosis)
Peripheral Blood Smear:
- Spherocytes
- Polychromasia
- RBC agglutination (in cold AIHA)
Reticulocyte Count:
- Elevated
Bilirubin:
- Elevated unconjugated (indirect) bilirubin
Lactate Dehydrogenase (LDH):
- Elevated
Haptoglobin:
- Decreased or absent
Direct Antiglobulin Test (DAT):
- Positive:
  - Warm AIHA: Positive for IgG (and sometimes complement)
  - Cold AIHA: Positive for complement (C3d) only
  - Drug-Induced Hemolytic Anemia: May be positive for IgG, complement, or both
  - HDFN: Positive for IgG
  - Hemolytic Transfusion Reactions: Positive for IgG, complement, or both

Key Terms

Autoimmune Hemolytic Anemia (AIHA): Anemia caused by autoantibodies against RBCs
Warm AIHA: IgG-mediated hemolysis at body temperature
Cold AIHA: IgM-mediated hemolysis at low temperatures
Paroxysmal Cold Hemoglobinuria (PCH): IgG-mediated hemolysis triggered by cold exposure
Drug-Induced Hemolytic Anemia: Hemolysis caused by drug-induced antibodies
Alloimmune Hemolytic Anemia: Hemolysis due to alloantibodies against foreign RBC antigens
Hemolytic Disease of the Fetus and Newborn (HDFN): Maternal antibodies attack fetal RBCs
Direct Antiglobulin Test (DAT): Test to detect antibodies or complement on RBCs
Spherocytes: Spherical red blood cells
Polychromasia: Increased number of reticulocytes
Cold Agglutinins: IgM antibodies that cause RBC agglutination at low temperatures

Hereditary Hemolytic

Hypoproliferative