Factor XI is a protein in the blood that plays a crucial role in the coagulation process, which helps stop bleeding. It is part of the intrinsic pathway of blood clotting and is activated by contact with negatively charged surfaces. Once activated, it helps convert another protein, Factor IX, into its active form, which further promotes the formation of a blood clot. Deficiencies or abnormalities in Factor XI can lead to bleeding disorders, such as hemophilia, where the blood does not clot properly. Conversely, excessive activity of Factor XI has been linked to an increased risk of thrombosis, where blood clots form inappropriately, potentially leading to serious health issues like stroke or heart attack.