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: Although acetylcholine receptor antibodies were not detectable , the time course was consistent with an autoimmune process .
### Output Text:
Although <span class="chemical">acetylcholine</span> receptor antibodies were not detectable , the time course was consistent with an autoimmune process . 