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: Heparin - induced thrombocytopenia ( HIT ) is an adverse immune - mediated reaction to heparin , resulting in platelet count decreases of more than 50 % .
### Output Text:
<span class="chemical">Heparin</span> - induced thrombocytopenia ( HIT ) is an adverse immune - mediated reaction to <span class="chemical">heparin</span> , resulting in platelet count decreases of more than 50 % . 