In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: The impact of immune - mediated heparin - induced thrombocytopenia type II ( HIT type II ) as a cause of thrombocytopenia after liver transplantation is not yet understood , with few literature citations reporting contradictory results .
### Output Text:
The impact of immune - mediated <span class="chemical">heparin</span> - induced thrombocytopenia type II ( HIT type II ) as a cause of thrombocytopenia after liver transplantation is not yet understood , with few literature citations reporting contradictory results . 