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: Thrombosis and thrombocytopenia that follow comprise the 2 hallmark traits of HIT , with the former largely responsible for significant vascular complications .
### Output Text:
Thrombosis and thrombocytopenia that follow comprise the 2 hallmark traits of HIT , with the former largely responsible for significant vascular complications . 