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: Blockade of histamine H1 receptors may reduce mortality in pertussis immunization - induced encephalopathy in mice .
### Output Text:
Blockade of <span class="chemical">histamine</span> H1 receptors may reduce mortality in pertussis immunization - induced encephalopathy in mice . 