What are HbA2 (Hemoglobin A2), Haemoglobin F (Fetal Hemoglobin), and HBH (Hemoglobin H) Inclusion Bodies?

Understanding Hemoglobin Variants: HbA2, Hemoglobin F, and HBH Inclusion Bodies

Hemoglobin variants are critical diagnostic markers in hematological disorders, with HbA2, Hemoglobin F, and HBH inclusion bodies each serving distinct clinical roles in identifying various hemoglobinopathies and thalassemias.

Hemoglobin A2 (HbA2)

• HbA2 is a normal minor hemoglobin variant consisting of two alpha-globin chains and two delta-globin chains (α₂δ₂), constituting approximately 2-3% of total hemoglobin in healthy adults 1.

• HbA2 is not detected in neonates but increases to its normal adult levels as children age 1.

• HbA2 measurement is crucial for diagnosing beta-thalassemia trait, where levels are typically elevated above 3.5% 2.

• Decreased HbA2 levels (below normal range) can be observed in:

  • Alpha thalassemia (range 1.39% to 2.43%) 3
  • Delta thalassemia (approximately 1.70-1.77%) 3
  • Iron deficiency anemia (lowest levels correlate with most severe anemia) 4
  • Hemoglobin H disease 4

• HbA2 quantification is typically performed using:

  • High-performance liquid chromatography (HPLC)
  • Capillary zone electrophoresis (CZE)
  • These methods may yield slightly different values, with HPLC typically showing higher values than CZE 2.

Hemoglobin F (Fetal Hemoglobin)

• Hemoglobin F consists of two alpha-globin chains and two gamma-globin chains (α₂γ₂) 1.

• HbF is the predominant hemoglobin in neonates, comprising 60-95% of total hemoglobin at birth 1.

• HbF levels normally decline after birth to less than 1% in older children and adults 1.

• Elevated HbF levels in adults can indicate:

  • Hereditary persistence of fetal hemoglobin 4
  • Beta-thalassemia (particularly transfusion-dependent beta-thalassemia) 5
  • Delta-beta thalassemia 3

• HbF levels have been shown to correlate with disease severity in transfusion-dependent beta-thalassemia (p<0.05), with higher levels generally associated with milder disease 5.

• Structural variants of HbF (such as Hb F-M-Osaka and Hb F-M-Fort Ripley) can occur due to mutations in gamma-globin genes, similar to mutations causing adult hemoglobin variants 6.

Hemoglobin H (HbH) Inclusion Bodies

• HbH inclusion bodies are characteristic findings in Hemoglobin H disease, a moderate to severe form of alpha thalassemia 4.

• HbH disease is characterized by:

  • Low HbA2 levels 4
  • Presence of HbH, which consists of four beta-globin chains (β₄) formed due to the relative excess of beta chains when alpha chain production is severely reduced 6.

• Alpha thalassemia is a common cause of hydrops fetalis, particularly in Southeast Asian populations, where it accounts for 28-55% of non-immune hydrops fetalis cases 6.

• Diagnosis of HbH disease involves:

  • Hemoglobin electrophoresis showing the presence of HbH
  • Peripheral blood smear examination revealing HbH inclusion bodies
  • Molecular genetic testing to identify alpha-globin gene deletions or mutations 6

• Parents of affected individuals can be screened by evaluating the mean cell volume (MCV), which will be <80 fL in thalassemia carriers 6.

Clinical Significance and Diagnostic Approach

• Quantification of these hemoglobin variants is essential for diagnosing various hemoglobinopathies and thalassemias 2.

• The diagnostic approach typically includes:

  • Complete blood count with red cell indices
  • Hemoglobin electrophoresis or HPLC to quantify different hemoglobin variants
  • Molecular genetic testing when specific mutations are suspected 6

• In cases of suspected alpha thalassemia with hydrops fetalis, DNA testing for alpha-thalassemia should be performed, especially if parental MCV is <80 fL 6.

• Abnormal hemoglobin variants can cause various clinical manifestations, including anemia, cyanosis, and in severe cases, hydrops fetalis and fetal demise 6.