Subunit concentrations. Mean ± SD concentration of the FXIII-A₂ subunit (A) A₂B₂ tetramer (B), and B-subunit (C) are shown. FXIII-A₂ subunit and FXIII-A₂B₂ tetramer concentrations increased after rFXIII administration. Because the FXIII-B subunit functions as a carrier protein for the FXIII-A₂ subunit, administration of rFXIII resulted in decreased FXIII-B subunit because of the rapid binding of rFXIII to free FXIII-B subunit.