in the clotting of blood, also known as plasma thromboplastin antecedent, a component of the contact system that is absorbed from plasma and serum by glass and similar surfaces. Its active form, factor XIa (EC 22.214.171.124), is a serine proteinase converting factor IX to factor IXa. Deficiency of factor XI results in a hemorrhagic tendency; is caused by an autosomal recessive gene.
Synonym(s): plasma thromboplastin antecedent
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