Factor XIIa

**Definition:** Activated form of factor XII. In the initial event in the intrinsic pathway of blood coagulation, kallikrein (with cofactor HIGH MOLECULAR WEIGHT KININOGEN) cleaves factor XII to XIIa. Factor XIIa is then further cleaved by kallikrein, plasmin, and trypsin to yield smaller factor XII fragments (Hageman-Factor fragments). These fragments increase the activity of prekallikrein to kallikrein but decrease the procoagulant activity of factor XII.

**Tree numbers:** - D08.811.277.656.300.760.324 - D08.811.277.656.959.350.324 - D12.776.124.125.450.324 - D23.119.450.324

**Synonyms:** - Factor 12A - Activated Factor XII - Factor XII, Activated - Blood Coagulation Factor XII, Activated - Prekallikrein Activator - Factor Twelve A - Coagulation Factor XIIa