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: However , for recipients of organ transplantation , removing the inciting agent is not without the attendant risk of precipitating acute rejection and graft loss .
### Output Text:
However , for recipients of organ transplantation , removing the inciting agent is not without the attendant risk of precipitating acute rejection and graft loss . 