Final answer:
Neurons, glial cells, and neurotransmitters are three environmental agents that signal danger.
Step-by-step explanation:
Three environmental agents that signal danger are neurons, glial cells, and neurotransmitters. The environmental agents that signal danger include PAMPs (pathogen-associated molecular patterns), DAMPs (damage-associated molecular patterns), and cytokines. These are all chemical signals that cells use to communicate and coordinate a response to threats. PAMPs and DAMPs are recognized by cells of the innate immune system through pattern recognition receptors (PRRs) such as Toll-like receptors, triggering an immune response. This recognizes patterns commonly found on pathogens or cellular damage. Cytokines are released in response to these signals and act as messengers to promote inflammation and recruit immune cells to sites of infection or damage, contributing to the innate and adaptive immune responses.