FXIII

Factor XIII (FXIII), also known as fibrin-stabilizing factor, is a zymogen in blood that stabilizes forming clots by cross-linking fibrin strands. In plasma, FXIII circulates as a heterotetramer (FXIII-A2B2) composed of two catalytic A subunits and two regulatory B subunits. Activation occurs when thrombin cleaves an activation peptide on the A subunits in the presence of calcium, causing dissociation of the B subunits and formation of the active enzyme FXIIIa, a calcium-dependent transglutaminase.

FXIIIa catalyzes the formation of covalent bonds between glutamine and lysine residues, reinforcing the fibrin network. This cross-linking increases clot stability and resistance to fibrinolysis. FXIIIa also cross-links other proteins, such as collagen, fibronectin, and α2-antiplasmin, contributing to tissue repair and wound healing. In addition to plasma FXIII, a cellular form of FXIII-A exists in platelets and monocytes, providing localized activity at injury sites.

Clinical aspects: FXIII deficiency is rare and can be congenital (often autosomal recessive) or acquired. Deficiency leads to bleeding that may be milder in early tests but severe clinically, including umbilical stump bleeding, wound dehiscence, mucosal bleeding, intracranial hemorrhage, and poor wound healing. Routine coagulation tests (PT, aPTT) are typically normal; specific FXIII activity or antigen assays are required for diagnosis. Management involves FXIII concentrates (plasma-derived or recombinant) for treatment of acute bleeding and prevention of recurrent hemorrhage. Prophylaxis may be necessary in severe congenital deficiency.

Overall, FXIII plays a key role in clot maturation and tissue integrity, extending its influence beyond coagulation to wound healing and vascular remodeling.