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 purpose of our study was to determine the cause of the patients ' reactions , the immunological mechanisms involved and whether these patients would be able to tolerate any kind of corticoid .
### Output Text:
The purpose of our study was to determine the cause of the patients ' reactions , the immunological mechanisms involved and whether these patients would be able to tolerate any kind of corticoid . 