Haemophilia A and B
- Lesion
Boys born without enough factor VIII or IX bleed deep into joints and muscles. Their platelets work, so small cuts are not the problem.
What it is
Haemophilia A (factor VIII deficiency) and haemophilia B (Christmas disease, factor IX deficiency) are X-linked recessive disorders that affect mostly males; heterozygous women occasionally bleed because of unfavourable X inactivation. Factors VIII and IX form the tenase complex that activates factor X on platelet surfaces, so their absence cripples thrombin generation. Severity tracks residual factor activity. Bleeding is deep and delayed: haemarthroses, muscle haematomas and bleeding after surgery or dental extraction, with petechiae characteristically absent.
Structure
- Joints: blood-stained synovium, haemosiderin-laden macrophages, synovial hypertrophy and fibrosis, cartilage destruction (haemophilic arthropathy).
- Muscles: large intramuscular haematomas that can compress nerves (compartment syndrome).
Step by step
- A mutation (often an inversion) abolishes or reduces factor VIII, or a mutation affects factor IX.
- The VIIIa–IXa tenase complex cannot efficiently activate factor X on the platelet surface.
- Thrombin burst is too small to make a stable fibrin clot.
- A platelet plug forms but breaks down: delayed and prolonged bleeding.
- Bleeding into weight-bearing joints recurs and destroys cartilage.
Haemophilia A, haemophilia B and von Willebrand disease
| Disorder | Missing factor | Inheritance | Tests | Main bleeding |
|---|---|---|---|---|
| Haemophilia A | Factor VIII | X-linked recessive | Long PTT, normal PT and platelets | Joints, muscles, after surgery |
| Haemophilia B (Christmas disease) | Factor IX | X-linked recessive | Long PTT, normal PT and platelets | Same as haemophilia A |
| von Willebrand disease | vWF (and secondarily VIII) | Usually autosomal dominant | Low ristocetin cofactor; PTT normal or long | Mucosal, heavy menses |
Clinical notes
- Bleeding at circumcision, joint swelling when a toddler starts walking, and intracranial bleeding after minor trauma.
- Treatment uses replacement factor, a bispecific antibody that mimics factor VIII and, for haemophilia B, gene therapy; cost limits access in low-resource settings.
Test yourself
Think of the answer, then tap the question to see it.
What coagulation test pattern do haemophilias A and B show?
Prolonged PTT with normal PT and normal platelet count.
Why are petechiae absent in haemophilia?
Platelets and the primary plug are normal; the defect is in fibrin formation, which affects deeper, delayed bleeding.
Why can a heterozygous woman with haemophilia A bleed?
By chance most of her cells inactivate the X chromosome with the normal factor VIII gene (unfavourable lyonization).
Elsewhere in DurgiPedia
Connections
Educational content on the mechanisms of disease; not a diagnostic or treatment guide and no substitute for a health professional's advice. No doses are given.
Sources: Hemophilia: MedlinePlus Genetics (MedlinePlus, US National Library of Medicine, 2026).
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Representative model, for educational purposes only.