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: All infants who were born to hepatitis B surface antigen - positive mothers also received hepatitis B immune globulin .
### Output Text:
All infants who were born to <span class="chemical">hepatitis B surface antigen</span> - positive mothers also received hepatitis B immune globulin . 