[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [General pathology](https://durgical.com/en/general-pathology/) › [Red cells and bleeding disorders](https://durgical.com/en/general-pathology/areas/red-cells/) › Haemophilia A and B

# 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.

Updated Nov 12, 2026

## Key points

- Prolonged PTT, normal PT, normal platelet count and bleeding time.
- About a third of cases of haemophilia A have no family history (new mutations).
- The most severe haemophilia A results from an inversion in the factor VIII gene.
- Recurrent haemarthroses cause chronic synovitis and crippling joint deformity.
- Inhibitory antibodies to infused factor develop in a fraction of severe haemophilia A patients.

## 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](https://durgical.com/en/conditions/haemophilia-a-and-b/) | Factor VIII | X-linked recessive | Long PTT, normal PT and platelets | Joints, muscles, after surgery |
| [Haemophilia B (Christmas disease)](https://durgical.com/en/conditions/haemophilia-a-and-b/) | Factor IX | X-linked recessive | Long PTT, normal PT and platelets | Same as haemophilia A |
| [von Willebrand disease](https://durgical.com/en/conditions/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.

## Memory aid

Boys, bleeding joints and a long PTT: eight is A, nine is B, and petechiae are not part of the story.

## 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).

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## Elsewhere in DurgiPedia

- [The coagulation cascade](https://durgical.com/en/physiology/the-coagulation-cascade/)
- [Coagulation tests: PT/INR, aPTT and platelets](https://durgical.com/en/physiology/coagulation-tests-pt-inr-aptt-and-platelets/)

## Connections

## Related 4

- [Mendelian inheritance: autosomal dominant, recessive and X-linked](https://durgical.com/en/general-pathology/mendelian-inheritance-autosomal-dominant-recessive-and-x-linked/)
- [Von Willebrand disease](https://durgical.com/en/general-pathology/von-willebrand-disease/)
- [Bleeding disorders: overview](https://durgical.com/en/general-pathology/bleeding-disorders-overview/)
- [Hemorrhage: petechiae, purpura, ecchymosis, hematoma, and what blood loss does](https://durgical.com/en/general-pathology/hemorrhage-petechiae-purpura-ecchymosis-hematoma-and-what-blood-loss-does/)

## Related conditions 2

- [Haemophilia A and B](https://durgical.com/en/conditions/haemophilia-a-and-b/)
- [Von Willebrand disease](https://durgical.com/en/conditions/von-willebrand-disease/)

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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](https://medlineplus.gov/genetics/condition/hemophilia/) (MedlinePlus, US National Library of Medicine, 2026).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/general-pathology/haemophilia-a-and-b/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/general-pathology/haemophilia-a-and-b/