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: Treatment of lethal pertussis vaccine reaction with histamine H1 antagonists .
### Output Text:
Treatment of lethal <span class="chemical">pertussis vaccine</span> reaction with <span class="chemical">histamine</span> H1 antagonists . 