Disclosed are antibodies and antigen-binding fragments that bind to a complementary peptide having an amino acid sequence that is encoded by the complement of a nucleotide sequence encoding thrombin or a portion thereof. Such antibodies and antigen-binding fragments can be agonists or antagonists of thrombin receptor-mediated events. Also disclosed are methods of using the agonist antibodies and antigen-binding fragments of the invention to activate the non-proteolytically activated thrombin receptor (NPAR) in a subject in need of such treatment. In addition, methods of using the antagonist antibodies and antigen-binding fragments of the invention to inhibit activation of the NPAR in a subject in need of such treatment are disclosed.