Anemia Hemolitik Autoimun (AIHA)

01Pathophysiology

AIHA is a disorder in which the immune system erroneously produces antibodies targeting antigens on the patient's own red blood cell membrane. The fundamental concept is a breakdown of self-tolerance at the B-cell level, resulting in autoantibody production that tags RBCs for destruction.

In warm AIHA (accounting for roughly 70-80% of cases), the autoantibodies are predominantly IgG class and react optimally at body temperature (37 degrees Celsius). These IgG-coated RBCs are recognized by Fc receptors on splenic macrophages. The macrophages do not always fully engulf the cell; instead, they often "nibble" portions of the RBC membrane away. This partial phagocytosis reduces the surface-area-to-volume ratio of the cell, transforming it from a biconcave disc into a spherocyte. This is why you see spherocytes on the peripheral smear, and it is also why the spleen is the primary site of destruction in warm AIHA (this is called extravascular hemolysis). Spherocytes are rigid and cannot deform to pass through the narrow splenic sinusoids, trapping them further and accelerating their destruction.

In cold AIHA (cold agglutinin disease), the autoantibodies are predominantly IgM class and bind to RBCs optimally at low temperatures (0-4 degrees Celsius, with clinical significance arising at temperatures below about 30 degrees Celsius in peripheral circulation). IgM is a potent activator of the classical complement pathway. When IgM binds to RBCs in cool peripheral circulation (fingertips, ears, nose), it fixes complement components up to C3b on the RBC surface. As the blood returns to the warmer central circulation, IgM detaches, but C3b remains. These C3b-opsonized RBCs are then cleared by hepatic Kupffer cells (which carry complement receptors), making the liver the primary site of destruction, not the spleen. If complement activation proceeds fully through the membrane attack complex (C5b-C9), intravascular hemolysis can also occur, which is why cold AIHA can present with hemoglobinuria.

Paroxysmal cold hemoglobinuria (PCH) is a rare subtype involving a biphasic IgG antibody (Donath-Landsteiner antibody) directed against the P antigen on RBCs. This antibody binds to RBCs in cold temperatures and fixes complement, but lysis occurs upon rewarming. It is most commonly seen in children following a viral infection and presents with episodic hemoglobinuria after cold exposure.

The connection between pathophysiology and symptoms is direct: destruction of RBCs releases hemoglobin, which is metabolized to unconjugated bilirubin (causing jaundice), while haptoglobin is consumed binding free hemoglobin (causing low serum haptoglobin). The bone marrow compensates by releasing immature red cells (reticulocytes), which explains the reticulocytosis. The marrow's demand for folate increases, and prolonged hemolysis can lead to folate deficiency if not supplemented.

02Classification and Clinical Manifestation

Drug-Induced AIHA Subtypes

03Diagnostic Workup

The Direct Antiglobulin Test (DAT), also known as the Direct Coombs test, is the cornerstone of the AIHA workup. It detects antibodies or complement proteins bound directly to the surface of the patient's RBCs. When a polyspecific DAT is positive, monospecific reagents are then used to determine whether the coating is IgG alone (favoring warm AIHA), C3d alone (favoring cold AIHA), or both IgG and C3d (seen in mixed-type AIHA or drug-induced forms).

Before concluding AIHA, you must first confirm that hemolysis is actually occurring. The classic hemolytic laboratory panel includes: elevated LDH, elevated indirect bilirubin, decreased haptoglobin, and reticulocytosis. Of these, haptoglobin is the most sensitive marker for hemolysis. A low haptoglobin with an elevated LDH and positive DAT essentially seals the diagnosis.

An important teaching point is the distinction between the Direct Coombs (detects antibodies on the RBC surface, used to diagnose AIHA) and the Indirect Coombs (detects free antibodies in the serum, used in blood bank crossmatching and prenatal screening). Exam questions that test this distinction will give a clinical vignette and ask which test to order. The answer for diagnosing AIHA is always the Direct Coombs.

On peripheral blood smear, warm AIHA classically shows spherocytes and polychromasia (the blue-tinged reticulocytes). Cold AIHA may show RBC agglutination (clumping) on the smear, particularly if the blood sample was not kept warm. This agglutination can also falsely elevate the MCV on an automated analyzer (a classic exam trick), because the machine reads clumped cells as single large cells. If you see a vignette with an "abnormally high MCV" in the setting of cold-triggered symptoms, think cold agglutinin disease with in vitro artifact.

When AIHA is diagnosed, the next step is to look for an underlying cause. In adults, this means screening for SLE (ANA, anti-dsDNA), lymphoproliferative disorders (flow cytometry, CT imaging), and performing a thorough medication review. In children with cold-type hemolysis, consider recent infection (Mycoplasma, EBV).

A point of caution: approximately 5-10% of AIHA cases are DAT-negative. If the clinical picture is convincing (hemolysis with no other explanation, spherocytes on smear, response to steroids) but the DAT is negative, the diagnosis is not excluded. These cases may involve low-affinity IgG or IgA antibodies not detected by standard DAT reagents. More sensitive testing (such as mitogen-stimulated DAT or flow cytometric DAT) may be needed.

04Management and Treatment

Warm AIHA treatment begins with corticosteroids. Prednisone at a dose of 1 to 1.5 mg/kg/day is standard. Most patients (60-80%) respond within 1 to 3 weeks. Once the hemoglobin stabilizes and the reticulocyte count begins to normalize, a slow taper is initiated over several months (typically 4-6 months total). Rapid tapering is a frequent cause of relapse. Patients who do not respond within 3 weeks, or who relapse during the taper, are considered steroid-refractory.

For steroid-refractory or relapsing warm AIHA, rituximab has become the preferred second-line agent. It depletes CD20-positive B cells, thereby reducing autoantibody production. The standard dose is 375 mg/m2 given intravenously once weekly for 4 consecutive weeks. Response rates are approximately 70-80%, with many patients achieving durable remission. Splenectomy remains an option for patients who fail rituximab, though it has fallen in the treatment algorithm compared to earlier practice. If splenectomy is planned, the patient must receive vaccinations against Streptococcus pneumoniae, Neisseria meningitidis, and Haemophilus influenzae type b at least two weeks before surgery.

A critical exam point: do not give steroids or perform splenectomy for cold agglutinin disease. Because the site of destruction in cold AIHA is predominantly the liver (Kupffer cells), not the spleen, splenectomy is ineffective. Furthermore, since cold agglutinin disease is driven by IgM (produced by B cells, not T-cell dependent in the same way), steroids have limited benefit. The correct first-line pharmacologic therapy is rituximab, and for patients who are refractory, sutimlimab, a monoclonal antibody targeting C1s in the classical complement pathway, is now available. Understanding this warm-versus-cold treatment distinction is a high-frequency testing target.

Transfusion in AIHA deserves attention. The autoantibody often reacts with all donor RBCs, making crossmatching difficult (the crossmatch will appear "incompatible"). However, you should never withhold transfusion from a hemodynamically unstable patient solely because crossmatching is difficult. The blood bank will issue the "least incompatible" units. In cold AIHA, use a blood warmer during transfusion to prevent further complement activation on transfused cells.

Folate supplementation (folic acid 1 mg/day) is given to all patients with chronic hemolysis because the bone marrow's increased erythropoietic activity consumes folate rapidly. Failure to supplement can result in a superimposed megaloblastic anemia, which blunts the reticulocyte response and worsens anemia further.

05Differential Diagnosis and Distractors

06Traps and High-Yield Pearls

The single most common trap with AIHA on licensing exams is the failure to distinguish warm from cold subtypes, because the treatment is fundamentally different. A vignette will describe a patient with hemolysis and a positive DAT, then ask for the next step. If the DAT is positive for IgG, you treat with steroids. If the DAT is positive for C3 only (indicating cold type), steroids and splenectomy are wrong answers, and you should look for cold avoidance or rituximab. Students who do not read the DAT pattern carefully will default to steroids for all AIHA, which is a designed distractor.

Another frequent gotcha involves spherocytes on smear. When a question shows spherocytes, many students jump to hereditary spherocytosis. The discriminator is the DAT: positive in AIHA, negative in hereditary spherocytosis. Any time you see spherocytes, you must determine the DAT result before choosing a diagnosis.

The transfusion dilemma is a tested concept. An unstable AIHA patient with a hemoglobin of 4 g/dL needs blood. The crossmatch is incompatible. The wrong answer is "do not transfuse." The correct answer is to transfuse with the least incompatible unit while the blood bank continues more detailed antibody workup. Never let a patient die because crossmatching is difficult.

Watch for the spuriously elevated MCV in cold agglutinin disease. If a vignette presents a "macrocytic anemia" with cold-triggered symptoms and RBC agglutination, the MCV is an artifact of clumping, and the true diagnosis is cold AIHA, not megaloblastic anemia.

Drug-induced AIHA is tested by asking what medication was recently started. Methyldopa is the classic inducer of true autoantibody formation (warm type, IgG positive DAT). Cephalosporins and quinidine cause immune-complex type hemolysis (C3 positive DAT). The management is to discontinue the offending drug, and hemolysis typically resolves over days to weeks.

Finally, remember that a negative DAT does not rule out AIHA in a compelling clinical scenario. DAT-negative AIHA accounts for roughly 5-10% of cases, and these patients still respond to the same treatments. This concept is tested to see whether you anchor too rigidly to a single laboratory result instead of integrating the full clinical picture.

The core competency being tested with AIHA is the ability to: (1) recognize immune-mediated hemolysis using the hemolytic labs plus DAT, (2) subtype it correctly by DAT pattern, (3) match the subtype to the correct treatment, and (4) avoid common management errors such as giving steroids for cold type or withholding transfusion in a critically ill patient.