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: Neutrophils from 14 ALF patients were stimulated via the complement receptors using zymosan opsonized with ALF or control serum .
### Output Text:
Neutrophils from 14 ALF patients were stimulated via the complement receptors using zymosan opsonized with ALF or control serum . 