Phase 1 · Understand the Body — Why the disease behaves the way it does.
The One-Line Truth
The whole disease in one chain
Everything on this page exists to explain this line.
Factor V Leiden is a genetic mutation that makes factor V resistant to protein C degradation, shifting the coagulation balance toward clot formation.
The Broken Security Guard
Imagine a nightclub where bouncers (factor V/Va) control how fast people enter through a door (prothrombin → thrombin conversion). A supervisor (protein C) normally walks around and taps specific bouncers on the shoulder at a marked spot on their uniform (arginine at position 506) to tell them to take a break. In Factor V Leiden, that marked spot is smudged or changed (glutamine replaces arginine), so the supervisor's tap doesn't register—the bouncer keeps working six times longer than normal. The club floods with people (thrombin builds up), the dance floor gets dangerously packed (fibrin clot forms), and eventually the fire exits get blocked (vein gets obstructed). The security system still works at the main entrance (antithrombin still clears some thrombin), but it can't stop the overzealous bouncer, so crowding happens anyway during busy nights (high-risk situations like flights or surgery).
American Society of Hematology (ASH) 2020 guidelines on venous thromboembolism: diagnosis, treatment, and prevention in adults · American College of Chest Physicians (ACCP) Evidence-Based Clinical Practice Guidelines on Antithrombotic Therapy and Prevention of Thrombosis, 9th edition (2012, reaffirmed 2019) · American College of Obstetricians and Gynecologists (ACOG) Practice Bulletin No. 197: Inherited Thrombophilias in Pregnancy (2018, reaffirmed 2022) · National Institute for Health and Care Excellence (NICE) Clinical Guideline NG89: Venous thromboembolism in adults – reducing the risk in hospital and after discharge (2018, updated 2019) · reviewed 2026-08-29