Basophilia

What is a basophil?

• Basophils are the least abundant white blood cells, making up <1% of circulating leukocytes.
• They are part of the granulocyte family, alongside neutrophils and eosinophils.
• Basophils are produced in the bone marrow and circulate in the blood.
• They play a role in immune responses, particularly in:
  • Allergic reactions (via histamine release)
  • Parasitic infections
  • Chronic inflammatory conditions
• Major functions:
  • Release histamine (causing vasodilation and inflammation)
  • Release heparin (prevents blood clotting)
  • Participate in allergic responses and defense against parasites
  • Secrete cytokines to recruit other immune cells
  • Involved in tissue repair and inflammation

Key features:

• Large cytoplasmic granules that stain dark blue or purple with basic dyes.
• Contain histamine, heparin, leukotrienes, and other mediators.
• Surface receptors for IgE (like mast cells).

Peripheral smear showing basophils, from Shutterstock with permission.

Schematic of a basophil, from Shutterstock with permission.

What is basophilia?

• Basophilia refers to an elevated absolute basophil count, typically >0.1 × 10⁹/L in adults (may vary slightly by lab).

Causes of basophilia

• Clonal/myeloproliferative disorders:
  • Chronic myeloid leukemia (CML) – most common and important cause¹
  • Polycythemia vera
  • Primary myelofibrosis
  • Essential thrombocythemia
• Reactive/non-clonal causes:
  • Allergic reactions (e.g., food allergy, urticaria, anaphylaxis)
  • Chronic inflammation (e.g., rheumatoid arthritis, ulcerative colitis)
  • Infections (especially viral or parasitic)
  • Endocrinopathies (e.g., hypothyroidism)
  • Post-splenectomy (transient)
  • Iron deficiency

Clinical implications

• Basophilia itself does not cause symptoms—symptoms, if present, relate to the underlying condition (e.g., symptoms of allergies, chronic inflammation, or blood cancers).
• Isolated mild basophilia is often nonspecific.
• Persistent or marked basophilia should raise suspicion for a myeloproliferative neoplasm, especially CML, and warrants further evaluation (e.g., CBC with differential, peripheral smear, BCR-ABL testing).

References:

1. How I investigate basophilia in daily practice. Int J Lab Hematol. 2020 Jun;42(3):237-245.
2. Basophils in peripheral blood and bone marrow. A retrospective review. Am J Med 1984 Mar;76(3):509-11.