Inhibits factor X (X(a)) directly and, in a Xa-dependent way, inhibits VIIa/tissue factor activity, presumably by forming a quaternary Xa/LACI/VIIa/TF complex. It possesses an antithrombotic action and also the ability to associate with lipoproteins in plasma (By similarity). Isoform alpha is expressed in heart and spleen; isoform beta in heart and lung. This inhibitor contains three inhibitory domains. The first domain interacts with VIIa and TF, the second one with Xa (By similarity).