Factor XIa
Pathway · detail · Haemophilia C APTT
Factor XI is activated by XIIa on a contact surface and, more importantly in vivo, by thrombin itself — a feedback loop that sustains factor IX activation once clotting is under way. It is the point at which the contact system stops being a laboratory curiosity and starts to matter clinically.
Traced from the start
Detail
- Activated by
- Factor XIIa, and by thrombin as feedback amplification
- Activates
- Factor IX to IXa
- Measured by
- APTT and a factor XI activity assay
- Drug target
- Factor XI inhibitors (abelacimab, milvexian, asundexian) aim to prevent thrombosis while sparing haemostasis, since XI deficiency bleeds only mildly
When it goes wrong
Factor XI deficiency (haemophilia C), autosomal and strongly enriched in Ashkenazi Jewish populations
Injury-related rather than spontaneous bleeding, especially after tonsillectomy, dental extraction and urological or obstetric surgery; bleeding correlates poorly with the factor level. Prolonged APTT with a normal PT
Practise this structure
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