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: TNF and PAF activity was found in the serum of respectively two and four of the cases , but not in the six controls .
### Output Text:
TNF and PAF activity was found in the serum of respectively two and four of the cases , but not in the six controls . 