Transferrin

Related Subjects: |Cardiac Troponins |Transferrin |Myoglobin |Heme |Globins

🔬 Transferrin is the major plasma glycoprotein responsible for transporting ferric iron (Fe³⁺) in the bloodstream. It binds free iron released from enterocytes, macrophages, or hepatocytes and delivers it safely to tissues (especially bone marrow for erythropoiesis, liver/spleen for storage). By sequestering iron, it also limits microbial access, contributing to innate immunity. Key lab marker: Total Iron-Binding Capacity (TIBC) ≈ transferrin concentration × 2 (two binding sites).

🧬 Structure of Transferrin

• Molecular weight: ~80 kDa single polypeptide chain (698 amino acids).
• Two homologous lobes: N-lobe and C-lobe (each ~40 kDa), connected by a flexible hinge region.
• Each lobe contains one high-affinity Fe³⁺ binding site (coordinated by 2 Tyr, 1 Asp, 1 His, and a carbonate/bicarbonate ion).
• Apo- vs holo-transferrin: Apo (iron-free) is open conformation; holo (iron-bound) is closed → more compact, higher receptor affinity.
• Glycosylation: Two N-linked oligosaccharide chains → influences half-life and receptor binding.

⚙️ Functions of Transferrin

• Iron transport & delivery 🚚: Binds Fe³⁺ with extremely high affinity (Kd ~10⁻²² M); prevents toxic free iron radicals.
• Cellular iron uptake 🔄: Holo-transferrin binds transferrin receptor 1 (TfR1) → clathrin-mediated endocytosis → endosome acidification (pH ~5.5) → iron release → apo-transferrin recycled to plasma.
• Iron homeostasis regulation ⚖️: TfR1 expression upregulated in iron deficiency (via IRP/IRE system); hepcidin ↓ ferroportin → ↓ iron export → ↓ transferrin saturation.
• Antimicrobial defense 🛡️: Iron sequestration (hypoferraemia of inflammation) starves pathogens; part of acute-phase response (↓ transferrin, ↑ ferritin/hepcidin).

📈 Regulation of Transferrin Levels & Saturation

• Low iron states 📉: ↑ transferrin synthesis (liver) → ↑ TIBC → low saturation (<16–20%) → maximizes iron uptake/mobilisation.
• High iron states 📈: ↓ transferrin synthesis → ↓ TIBC → high saturation (>45–50%) → risk of tissue deposition (hemochromatosis).
• Hepcidin axis: Master regulator → ↓ ferroportin → ↓ serum iron → ↓ transferrin saturation; inflammation ↑ hepcidin → functional iron deficiency.
• Other factors: Estrogen ↑ transferrin (pregnancy, OCP); chronic disease/inflammation ↓ transferrin (negative acute-phase reactant).

🩺 Clinical Significance & Interpretation

• Iron deficiency anaemia (IDA) 🔴:
  • ↑ transferrin / ↑ TIBC
  • ↓ transferrin saturation (<16%)
  • ↓ ferritin (<30 μg/L, often <15)
• Iron overload / Haemochromatosis 🟠:
  • ↑ transferrin saturation (>45–50%, often >60%)
  • ↑ ferritin (>300 μg/L men, >200 women)
  • Genetic testing (HFE C282Y/H63D)
• Anaemia of chronic disease/inflammation (ACD) 🟡:
  • ↓ transferrin / ↓ TIBC
  • Normal/low saturation
  • ↑ ferritin (acute-phase reactant)
• Combined IDA + ACD: Low TIBC + very low saturation + moderately ↑ ferritin.
• Transferrin saturation (TSAT): Serum iron ÷ TIBC × 100; key for diagnosis/monitoring (target 20–45% in CKD anaemia).

🔬 Lab Tests & Interpretation (2026)

• Serum transferrin: Direct immunoassay (normal 2.0–3.6 g/L).
• TIBC: Indirect measure (normal 45–80 μmol/L).
• TSAT: Most sensitive early marker of iron status.
• Alongside: Ferritin (storage), serum iron, CRP/ESR (inflammation), soluble TfR (tissue iron need).

Teaching Point 🩺 Transferrin = plasma iron shuttle (2 Fe³⁺ per molecule) → prevents toxicity + delivers to tissues via TfR1 endocytosis. ↑ transferrin/TIBC + ↓ saturation → iron deficiency. ↓ transferrin + high saturation + ↑ ferritin → overload. ↓ transferrin + normal/low saturation + ↑ ferritin → inflammation/chronic disease. Hepcidin links it all: ↑ hepcidin → ↓ iron availability → functional deficiency despite stores.

📚 References (Feb 2026)

• Ganz T. Hepcidin and iron regulation. Physiol Rev 2025 update.
• WHO/CDC Iron Biomarkers Guidelines (2024–2026).
• Andrews NC. Iron homeostasis pathways. Blood 2025 review.
• Recent: TfR1-targeted therapies in cancer (J Hematol Oncol 2026).