Factor XI triggers the middle phase of the intrinsic pathway of blood coagulation by activating factor IX. Selective cleavage of Arg-|-Ala and Arg-|-Val bonds in factor IX to form factor IXa. Inhibited by SERPINA5. Homodimer; disulfide-linked. Forms a heterodimer with SERPINA5. After activation the heavy and light chains are also linked by a disulfide bond (By similarity). Activated by factor XIIa (or XII), which cleaves each polypeptide after Arg-387 into the light chain, which contains the active site, and the heavy chain, which associates with high molecular weight (HMW) kininogen. N-glycosylated on both chains. N-glycosylated sites mainly consist of nonfucosylated sialylated biantennary (in high abundance) and/or triantennary (in low abundance) complex structures. Defects in F11 are the cause of factor XI deficiency in Japanese black cattle. It is a hereditary mild bleeding disorder with an autosomal recessive mode of inheritance. Belongs to the peptidase S1 family. Plasma kallikrein subfamily.