Autoimmune Hemolytic Anemia

Warm AIHA (IgG-mediated) Mechanism

• Autoantibody binding: IgG antibodies (predominantly IgG1 and IgG3 subclasses, which are most complement-fixing) bind to RBC surface antigens, most commonly the Rh antigen complex (particularly D, C, and E antigens) or other cryptic red cell epitopes. These IgG antibodies are typically present at 37°C and optimally activate complement at body temperature.
• Opsonization and extravascular hemolysis: IgG-coated RBCs are recognized by Fc receptors (particularly FcγRII and FcγRIII) on splenic macrophages, neutrophils, and monocytes. This binding leads to ADCC—the immune cells engulf IgG-opsonized RBCs or phagocytose them entirely, resulting in extravascular hemolysis primarily in the spleen (but also liver and bone marrow). The spleen becomes enlarged from the increased workload of processing antibody-coated RBCs. Importantly, IgG-mediated warm AIHA is typically direct antiglobulin test (DAT/Coombs) positive for IgG with or without complement.
• Complement activation: While IgG is less efficient at complement activation than IgM, some IgG subclasses (IgG1 and IgG3) can activate the classical complement cascade. C3b and C4b deposition on RBC surfaces may enhance phagocytosis through complement receptors on macrophages (CR1 and CR3). In some warm AIHA cases, sequential complement deposition can lead to C5b-9 membrane attack complex formation and intravascular hemolysis (less common but more acute presentation).
• Splenic sequestration and partial phagocytosis: The spleen can partially phagocytose antibody-coated RBCs, creating spherocytes—dense, rigid cells lacking central pallor. These cells are osmotically fragile and eventually completely lysed. This explains why spherocytes are the characteristic RBC morphology in warm AIHA.

Cold AIHA (IgM-mediated) Mechanism

• Cold-reactive antibody binding and complement fixation: Cold agglutinins are IgM antibodies that optimally bind RBC antigens (primarily Ii antigen—a structure on glycoproteins and glycolipids) at temperatures below 37°C, particularly in peripheral circulation where temperatures are lower. IgM is the most efficient activator of the classical complement pathway; a single IgM molecule binding to two adjacent RBC antigens can activate C1q, triggering the entire complement cascade.
• Complement-mediated intravascular hemolysis: In contrast to warm AIHA, IgM-mediated complement activation leads to intravascular hemolysis as the primary mechanism. Sequential complement activation generates C3b, C4b, and eventually C5b-9 membrane attack complex (MAC), which creates pores in the RBC membrane, causing osmotic lysis and hemoglobinuria. Cold AIHA patients classically present with dark urine, hemoglobinuria, and acute hemolytic episodes triggered by cold exposure.
• Absence of RBC sequestration in the spleen: Critically, complement-coated RBCs are recognized by complement receptors (CR1) on liver macrophages and hepatic sinusoids rather than splenic macrophages. Interestingly, if C3b is partially cleaved to C3d during the hemolytic process, the RBCs may not be completely lysed and can re-enter circulation after complement is cleared—allowing these cells to survive longer. This creates a chronic hemolytic pattern in some cold AIHA patients rather than acute crisis.
• Red cell agglutination: IgM antibodies can cross-link multiple RBCs, particularly in the microvasculature of acral areas exposed to cold. This causes RBC agglutination, leading to microvascular obstruction and tissue ischemia in fingers, toes, ears, and nose—explaining the acral cyanosis and cold-induced symptoms characteristic of cold AIHA.

Genetic and Immunologic Predisposition

• Loss of immune tolerance: The fundamental defect involves breakdown of central or peripheral tolerance. Impaired regulatory T cell (Treg) function and reduced suppression of autoreactive B cell clones allow production of anti-RBC antibodies. HLA associations exist (strongest with HLA-DR4 in warm AIHA), suggesting genetic predisposition.
• T cell help dysfunction: Autoreactive helper T cells provide costimulation to autoreactive B cells, promoting their survival and differentiation into plasma cells producing anti-RBC autoantibodies. This T cell-dependent B cell activation explains the effectiveness of T cell-targeting therapies (e.g., corticosteroids, rituximab).
• Epitope spreading and molecular mimicry: Secondary AIHA associated with lymphoproliferative disorders may arise from clonal expansion of abnormal lymphocytes producing anti-RBC antibodies. Infections (particularly viral) can trigger epitope spreading where cross-reactive antibodies against pathogen antigens attack RBC surface antigens through molecular mimicry.

Consequences of RBC Destruction

• Hemolysis-induced organ damage: Massive intravascular hemolysis releases hemoglobin, methemoglobin, and heme, which damage endothelium and cause acute kidney injury (via heme precipitation in renal tubules), hepatocellular necrosis, and myocardial infarction from coronary vasospasm. Free hemoglobin oxidizes to methemoglobin, which absorbs light at 630 nm—producing the characteristic dark/cola-colored urine seen in severe cases.
• Compensatory erythropoiesis: The bone marrow responds to shortened RBC survival with marked erythropoiesis. However, if hemolysis is severe, the marrow cannot compensate, resulting in brisk reticulocytosis (often >10%) despite falling hemoglobin. Reticulocyte production index (RPI) exceeding 3 confirms appropriate bone marrow response; failure to see appropriate reticulocytosis suggests concurrent bone marrow failure.
• Extramedullary hematopoiesis: Severe chronic hemolysis triggers extramedullary hematopoiesis in the spleen and liver, causing hepatosplenomegaly and further splenic sequestration of antibody-coated cells—a self-perpetuating cycle.

Primary (Idiopathic) AIHA

• Accounts for approximately 50-70% of AIHA cases; no identifiable underlying disease despite thorough investigation. Diagnosis requires exclusion of secondary causes. The mechanism involves spontaneous breakdown of immune tolerance, likely from a combination of genetic predisposition (HLA associations) and environmental triggers (viral infections, medications) that remain clinically silent or unrecognized.

Secondary AIHA - Lymphoproliferative Disorders (Most Common)

• Chronic lymphocytic leukemia (CLL) is the most common hematologic association, occurring in 10-35% of CLL patients. The leukemic B cells may directly produce anti-RBC antibodies. AIHA can be the presenting manifestation of occult CLL. Lymphoma (particularly Hodgkin lymphoma) accounts for 5-10% of secondary AIHA; malignant lymphocytes or altered immune regulation predispose to autoimmunity.
• Autoimmune lymphoproliferative syndrome (ALPS) and other inherited immunodeficiencies present with AIHA and characteristic other autoimmune phenomena due to defective Fas-mediated apoptosis of lymphocytes.

Secondary AIHA - Autoimmune and Connective Tissue Diseases

• Systemic lupus erythematosus (SLE) is the most common non-hematologic association, occurring in 5-10% of SLE patients. Anti-RBC antibodies occur alongside other autoantibodies (anti-DNA, anti-Smith, etc.). AIHA may precede SLE diagnosis by months to years.
• Antiphospholipid syndrome (APS) and Sjögren syndrome also present with AIHA as part of their autoimmune manifestations. Rheumatoid arthritis, thyroiditis, and idiopathic thrombocytopenic purpura (Evans syndrome—concurrent AIHA and ITP) occur with increased frequency.

Secondary AIHA - Infections

• Mycoplasma pneumoniae is a classic cause of cold AIHA, occurring in 3-10% of infected patients, particularly in younger adults. M. pneumoniae produces IgM cold agglutinins against the Ii antigen.
• Infectious mononucleosis (EBV), cytomegalovirus (CMV), hepatitis C, and HIV are associated with AIHA. These viruses trigger polyclonal B cell activation or molecular mimicry leading to anti-RBC antibody production. Syphilis (secondary) produces Wassermann antibodies, a type of IgM cold agglutinin causing cold AIHA.

Secondary AIHA - Medications

• Methyldopa (alpha-methyldopa) is the prototypical drug-induced AIHA, occurring in 0.8% of exposed patients. Methyldopa acts as a hapten or alters RBC membrane antigens, eliciting IgG antibodies. AIHA develops 3-12 months after initiation and persists weeks to months after drug discontinuation.
• Penicillins and cephalosporins (particularly high-dose IV penicillin) cause AIHA through epitope modification; antibodies form against penicillin-RBC conjugates. Quinine, sulfonamides, NSAIDs, and fluoroquinolones rarely cause immune-complex-mediated AIHA.

Secondary AIHA - Malignancies (Non-hematologic)

• Solid tumors (ovarian, gastric, lung cancer) rarely present with AIHA, likely through immune dysregulation or cross-reactive antigens between tumor and RBCs.

Secondary AIHA - Other Causes

• Transfusion-related AIHA develops following RBC transfusion in previously alloimmunized patients or those who develop allo-immunization. Post-transplant AIHA occurs after bone marrow or solid organ transplantation. Thermoagglutinin disease is an extremely rare variant associated with paroxysmal nocturnal hemoglobinuria (PNH).

Symptoms Related to Anemia and Compensatory Responses

• Fatigue and dyspnea are the most common presenting complaints, resulting from decreased oxygen-carrying capacity and compensatory increased cardiac output. Fatigue is often gradual in warm AIHA (chronic, low-grade hemolysis) but acute and severe in cold AIHA or severe warm AIHA (acute massive hemolysis).
• Syncope, presyncope, and chest pain reflect critical anemia with decreased cerebral and coronary perfusion, particularly if hemoglobin drops below 7 g/dL. These symptoms indicate need for urgent intervention.
• Jaundice (icterus) develops when unconjugated hyperbilirubinemia exceeds 2-3 mg/dL. In warm AIHA, jaundice reflects extravascular hemolysis with bilirubin production from Hb catabolism by splenic macrophages. Jaundice develops over days to weeks in chronic warm AIHA but can appear acutely in severe cases.
• Dark urine (hemoglobinuria or myoglobinuria) is characteristic of intravascular hemolysis seen in cold AIHA or severe warm AIHA with complement-mediated lysis. The urine appears cola or tea-colored due to myoglobin and hemoglobin. Hemoglobinuria indicates urgent risk of acute kidney injury.

Symptoms Related to Cold Exposure (Cold AIHA-specific)

• Acral pain, numbness, and cyanosis occur in fingers, toes, ears, and nose upon cold exposure due to RBC agglutination in microvasculature and microvascular obstruction. Patients report Raynaud-like symptoms triggered by cold temperatures.
• Livedo reticularis (lacy, net-like skin pattern) may develop from chronic microvascular obstruction.
• Hemoglobinuria after cold exposure confirms the diagnosis; patients note dark urine after handling cold objects or exposure to cold weather.

Physical Examination Findings

Important Clinical Variants

Step 1 — establish that hemolysis is present

• CBC with reticulocyte count: normocytic (or slightly macrocytic) anemia with brisk reticulocytosis; reticulocyte production index >3 confirms an appropriate marrow response.
• Hemolysis panel: elevated LDH, elevated indirect (unconjugated) bilirubin, and a low or undetectable haptoglobin. Haptoglobin is the most specific of the three; markedly depressed haptoglobin plus high LDH strongly supports hemolysis.
• Urine studies: hemoglobinuria (dipstick positive for blood, few RBCs on microscopy) and later hemosiderinuria indicate an intravascular component.
• Peripheral smear: spherocytes (partial phagocytosis by splenic macrophages) in warm AIHA; RBC agglutination/clumping at room temperature in cold agglutinin disease. Agglutination causes a spuriously low RBC count and a falsely elevated MCV and MCHC on automated counters — warming the specimen to 37°C corrects the artifact.

Step 2 — confirmatory test

• Direct antiglobulin test (DAT, direct Coombs) is the diagnostic gold standard and must be run with monospecific reagents, not just polyspecific: anti-IgG positive (± C3d) defines warm AIHA. C3d alone with negative IgG is characteristic of cold agglutinin disease (an isolated C3d pattern is also seen in paroxysmal cold hemoglobinuria, some drug-induced immune hemolysis, and occasional mixed-type AIHA); interpret it together with the cold agglutinin titer, thermal amplitude, and clinical context. The indirect Coombs detects free antibody in serum and predicts crossmatch difficulty but does not diagnose AIHA.
• Cold agglutinin titer performed at 4°C on a warm-transported sample; a titer of at least 1:64 in the setting of documented hemolysis supports cold agglutinin disease, and thermal amplitude near 30–37°C predicts clinical severity.
• Donath–Landsteiner test for paroxysmal cold hemoglobinuria: a biphasic IgG anti-P antibody that binds in the cold and lyses on rewarming.
• A minority of true AIHA is DAT-negative; the First International Consensus Meeting on AIHA (2020) advises repeat testing with more sensitive methods (microcolumn, flow cytometry, anti-IgA) before excluding the diagnosis.

Immediate stabilization

• Transfusion: never withhold RBCs from a hemodynamically unstable or ischemic patient because units are only "least incompatible" — the AABB and the 2020 International Consensus on AIHA endorse transfusing the best-matched (ABO/Rh and extended phenotype-matched) units while the antibody workup continues. Transfuse small aliquots with close monitoring; use a blood warmer and keep the patient and room warm in cold agglutinin disease.
• Supportive care: folic acid supplementation for the consumptive folate deficit, and pharmacologic thromboprophylaxis during active hemolysis unless contraindicated.

Warm AIHA — first line

• Corticosteroids: prednisone 1 mg/kg/day (or equivalent methylprednisolone IV in crisis), which blunt macrophage Fc-receptor–mediated clearance within days before reducing autoantibody production. Taper slowly over months; failure to respond in ~3 weeks defines steroid refractoriness.
• Anti-CD20 monoclonal antibody (rituximab): consensus recommendations support adding rituximab early to steroids in severe disease to improve durable response and reduce steroid exposure.

Warm AIHA — escalation and definitive therapy

• Second line: rituximab if not already used; splenectomy removes the principal site of IgG-opsonized RBC destruction and offers the highest durable remission rate of any single intervention.
• Third line immunosuppressants: azathioprine, mycophenolate mofetil, cyclophosphamide, cyclosporine, or danazol. IVIG is far less effective in AIHA than in ITP.
• Pre-splenectomy, give pneumococcal, meningococcal, and Haemophilus influenzae type b vaccines at least 2 weeks before surgery per CDC/ACIP asplenia recommendations.

Cold agglutinin disease

• Cold avoidance is the mainstay. Corticosteroids and splenectomy are largely ineffective because complement-coated cells are cleared in the liver — this is the classic exam distractor.
• Rituximab, alone or with bendamustine, targets the underlying clonal B cells. Sutimlimab, an anti-C1s antibody blocking the classical pathway, is FDA-approved for hemolysis in cold agglutinin disease; complement blockade mandates meningococcal vaccination.

Disease-related — emergencies

• Fulminant hemolytic crisis: precipitous hemoglobin fall with high-output cardiac failure, demand ischemia, and altered mentation; signaled by soaring LDH, unmeasurable haptoglobin, and hemoglobinuria. Requires ICU care and transfusion.
• Pigment nephropathy / acute kidney injury: free heme scavenges nitric oxide and precipitates in tubules; heralded by cola-colored urine with a dipstick positive for blood but few RBCs, plus a rising creatinine.
• Venous thromboembolism: hemolysis liberates free hemoglobin (NO depletion) and exposes procoagulant phosphatidylserine; DVT and pulmonary embolism are a leading cause of death in active AIHA. New dyspnea or pleuritic pain during a crisis is a PE until proven otherwise.
• Aplastic (parvovirus B19) crisis: transient arrest of erythroid precursors in a patient dependent on brisk erythropoiesis; the tell is a falling hemoglobin with a reticulocyte count near zero. Emergent transfusion ± IVIG.
• Acral ischemia/gangrene from cold-induced agglutination in cold agglutinin disease.

Disease-related — chronic

• Calcium bilirubinate (pigment) gallstones from chronic bilirubin load, presenting as biliary colic or cholecystitis.
• Folate depletion from sustained erythropoiesis, producing a paradoxical megaloblastic picture.
• Bleeding in Evans syndrome when thrombocytopenia coexists.

Treatment-related

• Corticosteroids: hyperglycemia, infection, osteoporosis, avascular necrosis of the femoral head, adrenal suppression on abrupt withdrawal.
• Rituximab: hypogammaglobulinemia, late-onset neutropenia, and hepatitis B reactivation — screen HBsAg and anti-HBc before dosing, as reactivation can cause fulminant hepatic failure; rare progressive multifocal leukoencephalopathy.
• Splenectomy: overwhelming post-splenectomy infection with encapsulated organisms (pneumococcus, meningococcus, Hib) — a true emergency requiring immediate empiric antibiotics for any fever; also portal/splenic vein thrombosis.
• Complement inhibition (sutimlimab): heightened risk of encapsulated organism, especially meningococcal, infection.
• Cyclophosphamide: myelosuppression, hemorrhagic cystitis, secondary malignancy.
• Transfusion: alloimmunization masked by the autoantibody, with delayed hemolytic transfusion reactions.

• Spherocytes + positive DAT = AIHA; spherocytes + negative DAT = hereditary spherocytosis. The DAT is the single discriminator; confirm HS with the EMA-binding flow cytometry or acidified glycerol lysis test rather than relying on osmotic fragility alone.
• Monospecific DAT is the money test. Anti-IgG (± C3d) → warm AIHA, extravascular, spleen. C3d only → cold agglutinin disease, complement-mediated, liver.
• Antigen associations examiners love: Mycoplasma pneumoniae → IgM anti-I; EBV/infectious mononucleosis → anti-i; paroxysmal cold hemoglobinuria → biphasic IgG anti-P with a positive Donath–Landsteiner test, classically in a child after a viral illness.
• The single best next step in a new adult AIHA is a DAT — and once warm AIHA is confirmed, peripheral blood flow cytometry, since CLL is the most common secondary cause and AIHA may be its presenting feature. In a lupus stem, AIHA can precede the diagnosis by years.
• Corticosteroids (± rituximab) are first-line in warm AIHA, with splenectomy and other immunosuppressants reserved for steroid-refractory or steroid-dependent disease per the 2020 First International Consensus and the BSH guideline — and all of these are near-useless in cold agglutinin disease, the most frequently tested distractor. Cold disease is managed by cold avoidance, rituximab-based therapy, or the anti-C1s agent sutimlimab.
• Do not withhold transfusion because the crossmatch is only "least incompatible." A hypoxic, ischemic patient gets blood; in cold disease, warm the blood and the patient.
• Automated counter artifact: agglutinated cold-antibody samples give a spuriously low RBC count with falsely high MCV and MCHC — warming the specimen normalizes it.
• Falling hemoglobin with a reticulocyte count near zero in chronic hemolysis means parvovirus B19 aplastic crisis, not worsening hemolysis.
• Drug clues: methyldopa is the prototype for warm, IgG-mediated drug-induced AIHA; high-dose penicillin acts by hapten-mediated RBC coating.