Factors affecting the evolution of factor XIa during blood coagulation.

Certain conditions affecting the evolution of factor XI-a activity during blood coagulation have been examined. Earlier data had indicated that calcium ion was not required for the conversion of factor XI to its activated form, but very little XI-a could be isolated from citrated or EDTA plasma, whether or not the plasma had been clotted by recalcification. Conversely, factor XI-a activity was identified in resin-decalcified plasma, again with or without recalcification. This confirmed that calcium is not required for the evolution of factor XI-a. This observation also permitted us to perform experiments which indicate that there is no obligatory participation of cellular elements in the evolution of factor XI-a during blood coagulation.