Factor XIII (aka Fibrin Stabilizing Factor, Laki-Lorand factor) is an enzyme (EC 2.3.2.13) of the blood coagulation system that crosslinks fibrin. When thrombin has converted fibrinogen to fibrin, the latter forms a proteinaceous network in which every E-unit is crosslinked to only one D-unit. Factor XIII is activated by thrombin into factor XIIIa; its activation into Factor XIIIa requires calcium as a cofactor.
A subunit (6p25-p24). The transglutaminase part; this adds an alkyl group to the nitrogen on a glutamine residue, which binds in turn with a lysine on the other chain.
B subunit (1q31-q32.1). This has no clear enzymatic activity, and may serve as a carrier for the A subunit.
Factor XIII deficiency may occur very rarely, and can cause a severe bleeding tendency. It has a long half life, ranging from 5-9 days.
DiaPharma distributes Affinity Biologicals factor XIII antibodies and deficiency plasma in the US.
Sources: DiaPharma, Wikipedia