Factor XI in haemostasis and thrombosis: Past, present and future

T he discipline of haemostasis and thrombosis has set an example, perhapsmore than other disciplines of Medicine, in discoveries based on observations of the so called"experiments of nature".During arelativelyshorttime of 13 years (1947)(1948)(1949)(1950)(1951)(1952)(1953)(1954)(1955)(1956)(1957)(1958)(1959)(1960), most coagulation factorsw ere identifiedinpatients whopresentedwith ableeding tendencyand were worked up by rather simple laboratoryassays. This era of discoveries was followedbythe characterization of the biochemical reactions involved in blood coagulation, delineation of the sequence of reactions in the coagulation pathwaysa nd more recentlyb yu nraveling the molecular genetic aspectsofthe variouscoagulation components. The objective of this reviewistodescribe some of the major developments in understandingthe role of factor (F)XI in haemostasis and thrombosis thath aveo ccurred over more than50years of research,and to highlight recent evidencewhich suggests that FXI deficiencycan have an antithrombotic effect.