Factor XIII is a transglutaminase that circulates in human blood as a heterotetramer of two A and two B subunits.

In the presence of fibrins, thrombin efficiently cleaves the R37–G38 peptide bond of each A unit within a XIII tetramer.

These bonds make the clot physically more durable and protect it from premature enzymatic degradation (fibrinolysis).

[1] In humans, plasmin, antithrombin and TFPI are the most relevant proteolytic inhibitors of the active factor XIIIa.

[1] Factor XIII of human blood is a heterotetramer of two A and two B linear polypeptides or "units".

[1] A subunits of human factor XIII are made primarily by platelets and other cells of bone marrow origin.

[2] Factor XIII deficiency, while generally rare, does occur, with Iran having the highest global incidence of the disorder with 473 cases.

The city of Khash, located in Sistan and Balochistan provinces, has the highest incidence in Iran, with a high rate of consanguineous marriage.

[3] Factor XIII levels are not measured routinely, but may be considered in patients with an unexplained bleeding tendency.

As the enzyme is quite specific for monocytes and macrophages, determination of the presence of factor XIII may be used to identify and classify malignant diseases involving these cells.