Autoimmune Hemolytic Anemia

Article written and reviewed by Dr Luca Moretti

Autoimmune hemolytic anemia is the most clinically relevant form of acquired hemolytic anemia and comprises a heterogeneous group of disorders characterized by premature red blood cell destruction mediated by autoantibodies directed against the patient’s own erythrocyte antigens. This condition creates an imbalance between red blood cell production and destruction, with important effects on erythropoiesis, erythrocyte survival and tissue oxygenation.

Clinical manifestations are extremely variable and range from mild or subclinical forms to acute, rapidly progressive and potentially fatal disease. The impact on quality of life and life expectancy depends closely on speed of onset, underlying cause, access to care and management of chronic immune mediated hemolytic complications.

Autoimmune hemolytic anemia may occur at any age but is more common in adults and older people. It is an important clinical problem in internal medicine, hematology and pediatric departments, where early recognition, prevention of complications and multidisciplinary treatment are fundamental elements of good clinical practice.

Etiology and Genetics

Autoimmune hemolytic anemia is caused by production of autoantibodies directed against red blood cell membrane antigens, without primary genetic mutations being involved in the process. The responsible autoantibodies are distinguished mainly by immunoglobulin class and thermal activity:

Three major categories of autoantibodies are currently recognized:

• Warm autoantibodies (IgG): active at 37°C, responsible for most adult cases and acting mainly in the spleen through extravascular hemolysis.
• Cold autoantibodies (IgM): active at low temperatures, from 4 to 30°C, typical of cold agglutinin disease and causing predominantly complement mediated extravascular hepatic hemolysis; an intravascular component may occur in more severe forms.
• Biphasic autoantibodies (Donath Landsteiner): rarer antibodies responsible for paroxysmal cold hemoglobinuria, acting in two phases through complement fixation in the cold followed by lysis at warm temperatures.

Autoimmune hemolytic anemia may be idiopathic, with no identifiable cause and more common in adults, or secondary to other conditions:

• Lymphoproliferative disorders, including chronic lymphocytic leukemia, non Hodgkin lymphomas and Waldenström macroglobulinemia
• Systemic autoimmune diseases, including systemic lupus erythematosus, rheumatoid arthritis and Sjögren syndrome
• Viral and bacterial infections, including Epstein Barr virus, CMV, HIV and Mycoplasma pneumoniae
• Medications, including penicillin, cephalosporins, methyldopa, quinidine, interferons and fludarabine
• Organ or bone marrow transplantation and primary or acquired immunodeficiencies

The disorder is not genetically inherited. It develops because of loss of immune tolerance to red blood cell antigens. Underlying diseases and modifying factors may profoundly influence the clinical course and response to treatment.

The prevalence of primary and secondary forms varies considerably among geographic areas and age groups, as does the distribution of the autoantibody subtypes involved.

Pathogenesis and Pathophysiology

Understanding the pathogenic mechanisms of autoimmune hemolytic anemia requires consideration of immune tolerance and self versus nonself recognition. The immune system normally does not produce antibodies against its own erythrocytes, but disruption of immune tolerance activates autoreactive B lymphocytes and generates a pathologic humoral response.

The presentation of the disease varies according to the type of autoantibody:

• In warm autoantibody (IgG) forms, antibodies bind erythrocytes at body temperature and promote phagocytosis by splenic macrophages, causing extravascular hemolysis.
• In cold autoantibody (IgM) forms, binding occurs at low temperatures with complement deposition and predominantly hepatic extravascular removal; intravascular lysis may contribute in more severe forms.
• In biphasic forms, red blood cell destruction is mediated by a two phase process of complement fixation and activation.

These differences explain the variability of clinical manifestations and complications among the different forms.

Binding of autoantibodies to the red blood cell surface causes opsonization and subsequent removal of erythrocytes from circulation through the reticuloendothelial system, mainly in the spleen and liver. Red blood cell destruction is responsible for hemolytic anemia, with release of hemoglobin, increased indirect bilirubin and activation of compensatory erythropoiesis.

Sensitized erythrocytes may also undergo intravascular lysis, particularly in forms with intense complement activation, causing hemoglobinuria and a risk of acute kidney injury. Severity of hemolysis depends on antibody titer, complement activation and splenic function.

Another key pathophysiologic feature is possible association with systemic immune hyperactivity and the risk of involvement of other blood cell lines, including thrombocytopenia and neutropenia, in secondary forms or systemic autoimmune syndromes.

Finally, persistent red blood cell destruction causes chronic stimulation of erythropoiesis, with possible circulating erythroblasts, erythroid bone marrow hyperplasia and signs of compensated or decompensated chronic anemia.

Clinical Presentation

The clinical picture of autoimmune hemolytic anemia is extremely variable and depends closely on speed of onset, quantity and characteristics of the autoantibodies, underlying disorders and compensatory capacity of the bone marrow.

Warm autoantibody autoimmune hemolytic anemia

This form is the most common in adults and presents with anemia of variable severity with subacute or acute onset, marked fatigue, cutaneous and mucosal pallor, mild jaundice or jaundice due to indirect hyperbilirubinemia, splenomegaly and, in severe cases, tachycardia and dyspnea. The clinical picture may rapidly progress to organ failure if not treated promptly.

Onset may be sudden with acute hemolytic crises or insidious and chronic, especially in forms associated with lymphoproliferative or autoimmune disease.

Cold autoantibody autoimmune hemolytic anemia

This form is more typical of older adults and is characterized by chronic hemolysis often triggered or worsened by cold exposure. Patients have acrocyanosis, Raynaud phenomenon, episodic hemoglobinuria and jaundice. Anemia may vary in severity, and cold agglutinin disease is associated with increased thromboembolic risk. Kidney injury is not a typical general complication and may occur exceptionally with intense intravascular hemolysis.

Biphasic autoimmune hemolytic anemia

This rarer form, typical of children or adults with acute viral infections, presents with acute episodes of hemoglobinuria associated with pain, fever, severe anemia and risk of acute kidney failure.

Common manifestations and clinical complications:

Regardless of subtype, several manifestations may occur with variable frequency:

• Chronic jaundice from persistent hemolysis
• Splenomegaly and/or hepatomegaly
• Acute hemolytic crises triggered by infections, medications or stress
• Symptoms of chronic anemia, including fatigue, dyspnea and tachycardia
• Hemoglobinuria in cold and biphasic forms
• Increased thrombotic risk, especially in chronic forms and splenectomized patients
• Susceptibility to infections due to immunosuppression or splenectomy
• Other immune cytopenias, as in Evans syndrome

Overall, the natural history of autoimmune hemolytic anemia is intertwined with the underlying disorder and treatment strategies. Early diagnosis and prompt initiation of therapy are decisive for prognosis.

Investigations and Diagnosis

The diagnosis of autoimmune hemolytic anemia is based on a sequential and integrated process that begins with clinical and laboratory suspicion, proceeds to immunohematologic characterization and, when indicated, is completed by imaging and investigation of underlying causes. The goal is to distinguish the clinical forms accurately, differentiate them from other causes of hemolytic anemia and promptly identify patients at risk of complications.

Suspicion generally arises in the presence of normocytic or macrocytic anemia with reticulocytosis, increased indirect bilirubin, elevated lactate dehydrogenase (LDH), reduced or absent haptoglobin and a personal history of hemolytic episodes or autoimmune disease.

First line laboratory workup:

• Complete blood count: normocytic or macrocytic anemia, reticulocytosis, with white blood cell and platelet counts usually normal; other cytopenias may occur in associated immune syndromes.
• Peripheral blood smear: polychromasia, spherocytes, circulating erythroblasts and occasionally schistocytes.
• Hemolysis markers: increased indirect bilirubin, elevated LDH and reduced or absent haptoglobin.

Immunohematologic tests: Diagnosis is based on integration of the clinical and laboratory picture with the direct Coombs test, or DAT, which detects immunoglobulin or complement bound to erythrocytes; a minority of cases may be DAT negative with conventional methods:

• Direct Coombs test positive for IgG: typical of warm autoantibody forms
• Direct Coombs test positive for C3d: typical of cold autoantibody forms
• Indirect Coombs test: may support immunohematologic and transfusion assessment by identifying circulating antibodies, but is not the fundamental diagnostic test for AIHA

These findings, integrated with the clinical and laboratory picture, allow diagnosis in most cases.

Additional investigations: In uncertain cases or to identify secondary or complex forms, the following are indicated:

• Screening for autoimmune disease, including ANA, ENA, anti DNA antibodies and rheumatoid factor
• Testing for viral and bacterial infections, including EBV, CMV, HIV and Mycoplasma
• Evaluation for monoclonal gammopathies with serum protein electrophoresis and immunofixation
• Lymphocyte immunophenotyping when lymphoma or leukemia is suspected

Imaging and follow up investigations: These are essential for assessment of complications and treatment monitoring:

• Abdominal ultrasonography: assessment of splenomegaly, hepatomegaly and signs of chronic hemolysis
• Kidney function: especially in forms with hemoglobinuria
• Chest radiography and targeted imaging: when lymph node or splenic involvement is suspected

Differential diagnosis: Autoimmune hemolytic anemia should be distinguished from other forms of hemolytic anemia, including enzyme defects, membrane disorders and thrombotic microangiopathies, from hemoglobinopathies and, in secondary forms, from other autoimmune cytopenias.

Specialist consultation: In complex or refractory cases, hematology, immunology and, when necessary, rheumatology consultation is essential for multidisciplinary management and identification of targeted therapies.

Timely diagnosis, precise definition of the form and assessment of complications are indispensable foundations for personalized treatment and prevention of long term consequences.

Treatment and Prognosis

The treatment of autoimmune hemolytic anemia is based on a personalized strategy that considers severity of the clinical picture, speed of progression, underlying cause and presence of complications. The primary goals are to stop hemolysis, restore adequate hemoglobin levels, prevent organ injury, manage underlying causes and reduce the risk of relapse and complications. Treatment has evolved in recent decades with the introduction of new immunosuppressive and biologic therapies.

Immunosuppressive treatment in warm autoantibody forms: In warm antibody AIHA, first line treatment involves high dose corticosteroid therapy, using prednisone or methylprednisolone, with response in most cases. In patients with inadequate response or early relapse, the following is added or substituted:

• Rituximab, an anti CD20 antibody
• Second line immunosuppressants, including azathioprine, cyclosporine or mycophenolate mofetil
• Intravenous immunoglobulin, or IVIG, in selected cases, especially pediatric patients or as rescue therapy; efficacy is variable and IVIG is not routine first line treatment for warm AIHA

Treatment selection depends on severity of presentation, comorbidities and autoantibody subtype.

Transfusion therapy: Reserved for severe symptomatic anemia and always administered with the utmost caution because of the risk of alloimmunization and transfusion reactions. Red blood cell units should be selected carefully, seeking the greatest possible compatibility.

Supportive therapy: Management of complications and support of vital functions include:

• Hemodynamic support and management of acute organ failure
• Prevention and treatment of opportunistic infections during immunosuppression
• Prevention and treatment of thromboembolic complications
• Vitamin supplementation with folic acid

Splenectomy: May be considered in patients with refractory or relapsing warm antibody AIHA after assessment of therapeutic alternatives. Efficacy must be balanced against infectious and thrombotic risks, and the procedure should be preceded by appropriate vaccination.

Innovative therapies: Biologic and molecular strategies have recently been developed:

• Rituximab is the principal biologic therapy with established efficacy.
• Complement inhibitors: sutimlimab is an approved therapy for cold agglutinin disease in selected patients; other inhibitors have different indications or levels of evidence.
• Plasma exchange in severe acute refractory hemolytic crises.

Prognosis: The prognosis of autoimmune hemolytic anemia has improved with early diagnosis and new treatment strategies. Nevertheless, the risk of relapse and immune hemolytic complications persists, especially in secondary and refractory forms. Mortality is related mainly to infectious and thrombotic complications and failure to control acute hemolysis.

In patients with underlying disorders such as lymphoma or systemic autoimmune disease, prognosis also depends on the course of the primary disease.

Prevention of secondary forms involves control of underlying diseases, judicious use of immunomodulatory medications and clinical surveillance of individuals at risk.

Complications

The complications of autoimmune hemolytic anemia are the main cause of morbidity and mortality, especially in severe, relapsing cases or those associated with systemic disease. The risk of complications is directly proportional to the severity and speed of hemolysis, presence of comorbidities and management of immunosuppressive therapy.

Infectious complications: Immunosuppressive therapy and splenectomy significantly increase the risk of serious bacterial and viral infections:

• Bacterial infections caused by encapsulated organisms, including Streptococcus pneumoniae, Neisseria meningitidis and Haemophilus influenzae
• Opportunistic viral and fungal infections
• Reactivation of latent infections, including HBV, TB, CMV and HSV

Vaccination and clinical vigilance are essential.

Thromboembolic complications: Thrombotic risk is increased both by chronic hemolysis and by steroid use and splenectomy. Venous and arterial thromboembolic events, including DVT, pulmonary embolism and stroke, are more common in patients with AIHA than in the general population.

Renal complications: Massive hemolysis with hemoglobinuria may cause acute tubular injury and kidney failure, especially in cold and biphasic forms.

Hematologic complications: Autoimmune hemolytic anemia may be associated with other immune cytopenias, as in Evans syndrome. Development of pancytopenia instead requires investigation for an associated disorder or a distinct bone marrow mechanism.

Metabolic and nutritional complications: Prolonged corticosteroid therapy may cause weight gain, hyperglycemia, osteoporosis and other metabolic complications. Malnutrition or weight loss are not features of AIHA and require investigation of alternative causes.

Psychosocial and quality of life complications: Chronic disease, dependence on immunosuppressive medications, relapses and the risk of complications have a major impact on psychological well being and quality of life, especially in young and active patients.

Overall, management of complications in autoimmune hemolytic anemia requires balancing control of the underlying disease, infection prevention, monitoring of thrombotic complications and multidisciplinary care, with the aim of ensuring good survival and quality of life.

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