Hemophilia A and B

Hemophilia A and B are inherited bleeding disorders caused by a deficiency of a single clotting factor — factor VIII in hemophilia A and factor IX in hemophilia B. Both are X-linked recessive conditions, so they predominantly affect males, and both impair the propagation phase of coagulation, producing a tendency to bleed into joints, muscles, and other tissues. The two disorders are clinically similar and distinguished mainly by which factor is missing.

Definition

Hemophilia A is a congenital deficiency or dysfunction of coagulation factor VIII, and hemophilia B (Christmas disease) is a congenital deficiency or dysfunction of factor IX; both are X-linked recessive disorders that impair thrombin generation and cause a lifelong bleeding tendency proportional to the residual factor level.

Scope

This entry covers the genetic basis, the role of factors VIII and IX in coagulation, the spectrum of clinical severity defined by residual factor activity, the characteristic pattern of bleeding, and the conceptual basis of laboratory diagnosis. It treats hemophilia as a reference topic in hematology and does not provide dosing, product selection, or individualized management advice.

Core questions

• How do factor VIII and factor IX participate in the coagulation cascade, and why does losing either cause a similar bleeding phenotype?
• How does residual factor activity define mild, moderate, and severe disease?
• Why are hemophilia A and B X-linked, and how does that explain their inheritance pattern?

Key concepts

• X-linked recessive inheritance
• Factor VIII deficiency (hemophilia A)
• Factor IX deficiency (hemophilia B)
• Residual factor activity and severity grading
• Hemarthrosis and hemophilic arthropathy
• Inhibitor (alloantibody) development
• Carrier females

Mechanisms

Factors VIII and IX act together in the intrinsic pathway of the coagulation cascade: activated factor IX, with factor VIII as its cofactor, forms the tenase complex that activates factor X and amplifies thrombin generation. A deficiency of either factor disrupts this amplification step, so the initial platelet plug forms but is not reinforced by adequate fibrin, leading to delayed and prolonged bleeding. Because both genes lie on the X chromosome, the disorders follow X-linked recessive inheritance, with affected males and usually asymptomatic carrier females. Disease severity tracks with residual factor activity, and a recognized complication is the development of neutralizing alloantibodies (inhibitors) against the deficient factor. Mannucci and Tuddenham (2001) review this pathophysiology and its history.

Clinical relevance

Understanding hemophilia clarifies why a prolonged activated partial thromboplastin time with a normal prothrombin time points to a defect in the intrinsic pathway, and why bleeding is typically into joints and muscles rather than mucosal surfaces. This entry describes the disorder for reference; clinical decisions about factor replacement, prophylaxis, and inhibitor management follow specialist guidelines and are outside its scope.

Epidemiology

Hemophilia A is the more common of the two, with hemophilia B occurring several times less frequently; both are rare and affect populations worldwide without strong ethnic predilection. Severity distribution and prevalence estimates are discussed in the World Federation of Hemophilia guidelines (Srivastava et al., 2020).

Evidence & guidelines

The World Federation of Hemophilia Guidelines for the Management of Hemophilia, 3rd edition (Srivastava et al., 2020), is the principal international reference for diagnosis, severity classification, and care principles. This entry cites that guideline for orientation rather than reproducing its recommendations.

History

Hemophilia has been recognized since antiquity as a hereditary bleeding tendency, famously appearing among European royal families descended from Queen Victoria, which gave the condition its association with 'royal genes.' The twentieth century brought the distinction between factor VIII and factor IX deficiency — the latter described as Christmas disease — and later the cloning of both genes, which enabled molecular diagnosis and recombinant factor production. Mannucci and Tuddenham (2001) trace this arc from royal pedigrees to modern molecular therapy.

Key figures

• Pier Mannucci
• Edward Tuddenham
• Alok Srivastava

Seminal works

• mannucci-tuddenham-2001
• srivastava-2020

Frequently asked questions

What is the difference between hemophilia A and hemophilia B?

Hemophilia A is caused by a deficiency of clotting factor VIII and hemophilia B by a deficiency of factor IX. The two are clinically very similar because both factors act at the same amplification step of coagulation; they are distinguished by specific factor assays.

Why does hemophilia mostly affect males?

The genes for factors VIII and IX lie on the X chromosome, and the disorders are inherited in an X-linked recessive pattern. Males, having a single X chromosome, are affected when they inherit the variant, while females are usually carriers.