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-III). Also inhibits chymotrypsin, but in a glycosaminoglycan- independent manner. {ECO:0000269|PubMed:1939083, ECO:0000269|PubMed:32827448}. Peptides at the N-terminal of HC-II have chemotactic activity for both monocytes and neutrophils. . [Isoform 2]: Shows negligible inhibition, in vitro, of thrombin and tPA and no inhibition of factor Xa, in vitro. . This is the function of ENSG00000099937 (SERPIND1, serpin family D member 1).