Thrombin inhibitor activated by the glycosaminoglycans, heparin or dermatan sulfate. In the presence of the latter, HC-II becomes the predominant thrombin inhibitor in place of antithrombin III (AT) (By similarity). The N-terminal acidic repeat region mediates, in part, the glycosaminoglycan-accelerated thrombin inhibition. Different composition of the N-linked oligosaccharides appears to yield a 68-kDa and a 72-kDa form. Belongs to the serpin family.