Chemicals for immune response (histamines) and pain relief (opiods)
Neuropeptides, which include pain-regulating messengers released within the brain, such as endorphins
The neurotransmitters acetylcholine and serotonin
Prostaglandins, which are lipids that, among other functions, create the sensation of pain in spinal nerve cells
Stimulation by any of these agents is typically related to inflammation, dryness or other damage to the skin, mucous membranes or conjunctiva of the eye.