Autoimmune hemolysis (warm/cold)

Summary

Autoimmune hemolytic anemia (AIHA) is a normocytic, normochromic anemia caused by autoantibodies that bind red blood cells (RBCs) and lead to their early destruction. It is divided into two main types based on the temperature at which the antibody best reacts with RBCs:

• Warm AIHA – caused by IgG antibodies that bind RBCs at body temperature (~37°C). RBCs are destroyed mainly in the spleen (extravascular hemolysis). It is the most common form (~80% of cases).
• Cold AIHA – caused by IgM antibodies that bind RBCs in cold areas (fingers, toes, nose, ears, <4°C). IgM activates complement, causing intravascular hemolysis and RBC agglutination.

The hallmark diagnostic test is the direct Coombs test (DAT):

• Warm AIHA → DAT positive for anti-IgG (± C3).
• Cold AIHA → DAT positive for anti-C3 only.

Treatment differs sharply: warm AIHA responds to corticosteroids, while cold AIHA is treated mainly by cold avoidance and rituximab; steroids and splenectomy are not effective in cold AIHA.

Definition and Classification

AIHA = acquired hemolytic anemia caused by autoantibodies directed against the patient's own RBC surface antigens.

• Warm AIHA (~80%) – IgG antibody, optimal binding at 37°C, extravascular hemolysis (spleen).
• Cold AIHA (~20%) – IgM antibody, optimal binding at 0–4°C, intravascular hemolysis (complement-mediated).
• Mixed type – rare, features of both.
• Drug-induced – e.g., penicillins, cephalosporins, α-methyldopa.

Etiology

Warm AIHA

• Primary (idiopathic) – about half of cases.
• Secondary:
  • Autoimmune diseases → SLE (most classic), rheumatoid arthritis.
  • Lymphoproliferative disorders → CLL, non-Hodgkin lymphoma.
  • Drugs → α-methyldopa, penicillins, cephalosporins, NSAIDs.

Cold AIHA

• Primary (idiopathic) – chronic cold agglutinin disease (elderly).
• Secondary:
  • Infections (acute, self-limited) → Mycoplasma pneumoniae and EBV (infectious mononucleosis).
  • Lymphoproliferative disorders → Waldenström macroglobulinemia, lymphomas.

Pathophysiology

Warm AIHA – Extravascular hemolysis

• IgG binds RBC surface antigens (mainly Rh) at body temperature.
• The Fc portion of IgG is recognized by macrophages in the spleen.
• Macrophages partially "bite" the RBC membrane → loss of membrane → spherocyte formation.
• Spherocytes get trapped and destroyed in the spleen → splenomegaly, indirect hyperbilirubinemia.

Cold AIHA – Intravascular hemolysis

• IgM binds RBC antigens (mainly I antigen in Mycoplasma; i antigen in EBV) in cold peripheral parts of the body.
• IgM is a strong activator of complement (C3).
• Two outcomes:
  • Full complement activation (MAC, C5–C9) → intravascular hemolysis → hemoglobinemia and hemoglobinuria.
  • Partial activation (C3b coating only) → RBCs cleared in the liver (Kupffer cells).
• IgM is also large (pentamer) → causes RBC agglutination in cold areas (acrocyanosis, Raynaud-like).

Clinical Features

General features of hemolysis are seen in both forms: fatigue, pallor, jaundice, dark urine, splenomegaly (especially warm).

The key clinical differences:

• Warm AIHA – more chronic, gradual anemia. Often presents with jaundice and splenomegaly. May follow a relapsing-remitting course.
• Cold AIHA – symptoms triggered by cold exposure:
  • Acrocyanosis – blue/purple discoloration of fingers, toes, ears, nose that improves on warming.
  • Raynaud-like phenomenon.
  • Episodic dark urine (hemoglobinuria) after cold exposure.
  • Post-infectious cases (Mycoplasma, EBV) are usually acute and self-limited.

A useful spotter sign: a CBC tube left on the bench may show visible RBC clumps in cold AIHA, and the analyzer may report a falsely elevated MCV (clumps counted as one large cell).

Side-by-side comparison

For an extended reference grid covering etiology, lab findings, and complications, see the linked comparison: Warm vs Cold Agglutinin AIHA – full reference table.

Diagnosis

1. General hemolysis labs (both types)

• ↓ Hemoglobin, ↑ reticulocyte count (compensatory marrow response).
• ↑ Indirect bilirubin, ↑ LDH, ↓ haptoglobin.
• Cold AIHA additionally → hemoglobinemia, hemoglobinuria (intravascular hemolysis).

2. Peripheral blood smear

• Warm AIHA → spherocytes + polychromasia (reticulocytes). Looks identical to hereditary spherocytosis on smear — Coombs distinguishes them.
• Cold AIHA → RBC agglutination (irregular clumps), not the orderly stacked-coin rouleaux of myeloma.

Warm AIHA smear – spherocytes  |  Cold AIHA smear – RBC agglutination  |  Rouleaux (for contrast)

3. Direct Coombs test (DAT) – the diagnostic test

Detects antibody and/or complement already bound to the patient's RBCs.

• Warm AIHA → DAT positive for anti-IgG (± C3).
• Cold AIHA → DAT positive for anti-C3 only (IgM has fallen off the RBC at lab temperature, but complement remains attached).

4. Cold-specific tests

• Cold agglutinin titer – elevated.
• Indirect Coombs may detect free IgM in serum.

Differential Diagnosis

• Hereditary spherocytosis – spherocytes but Coombs negative; positive family history.
• G6PD deficiency – bite cells, Heinz bodies; triggered by oxidative stress; Coombs negative.
• Microangiopathic hemolytic anemia (MAHA) – TTP/HUS/DIC: schistocytes, thrombocytopenia, Coombs negative.
• Paroxysmal nocturnal hemoglobinuria (PNH) – Coombs negative; positive flow cytometry for CD55/CD59 deficiency.
• Hemolytic transfusion reaction – Coombs may be positive but in the setting of recent transfusion.
• Multiple myeloma rouleaux – stacked-coin appearance, NOT agglutination; no hemolysis labs.

Management

Warm AIHA

1. Corticosteroids (prednisone 1 mg/kg/day) – first-line. Effective in ~70–80%.
2. Rituximab – if steroid-refractory or relapsing.
3. Splenectomy – for steroid- and rituximab-refractory cases (removes the main site of RBC destruction).
4. Other immunosuppressants – azathioprine, cyclophosphamide, mycophenolate.
5. Treat the underlying cause – stop the offending drug, treat SLE/CLL.
6. Folate supplementation (high RBC turnover).
7. Transfusion only if life-threatening anemia (crossmatch is difficult — warm the patient and use the "least incompatible" unit).

Cold AIHA

1. Cold avoidance – keep extremities warm; warm IV fluids and blood products. This is the cornerstone.
2. Treat underlying cause – Mycoplasma/EBV cases usually self-resolve.
3. Rituximab – first-line drug therapy for chronic/severe cold agglutinin disease.
4. Plasmapheresis – useful in acute severe hemolysis (removes circulating IgM, which is intravascular).
5. Steroids and splenectomy are NOT effective in cold AIHA (hemolysis is in the liver, not spleen, and IgM is not steroid-responsive).

Complications

• Severe symptomatic anemia – fatigue, dyspnea, heart failure in elderly.
• Hemolytic crisis – especially in cold AIHA after cold exposure.
• Venous thromboembolism (VTE) – AIHA is a recognized hypercoagulable state.
• Pigment gallstones (chronic indirect hyperbilirubinemia).
• Transfusion difficulties – autoantibodies interfere with crossmatching.
• Post-splenectomy (warm AIHA) – risk of encapsulated organism infections; vaccinate against S. pneumoniae, H. influenzae, N. meningitidis.
• Long-term steroid toxicity – osteoporosis, diabetes, infection, Cushingoid features.

Mnemonics