Probably a definite answer to this question does not exist. Here are some hints from the below cited recent excellent comprehensive article.

Stinging hairs can be classified into two basic types: Urtica-type with the classical “hypodermic syringe” mechanism expelling only liquid, and Tragia-type expelling a liquid together with a sharp crystal. Stinging hairs are found in ca. 650 flowering plant species across five different plant families belonging to distantly related orders. In spite of their morphological similarity, stinging hairs show striking differences in detail. Since the middle of the 20th century, neurotransmitters (acetylcholine, histamine, serotonin) have been repeatedly detected in stinging hairs of Urticaceae, but recent analyses revealed high variability in their composition and content of neurotransmitters. These substances can explain some of the physiological effects of stinging hairs, but fail to completely explain neuropathic effects, pointing to some yet unidentified neurotoxin. Inorganic ions (e.g., potassium) are detected in stinging hairs and could have synergistic effects. Very recently, ultrastable miniproteins dubbed “gympietides” have been reported from two species of Dendrocnide, arguably the most violently stinging plant. Gympietides are shown to be highly neurotoxic, providing a convincing explanation for Dendrocnide toxicity. Many aspects of stinging hairs in plants are well understood, but the details of their chemistry and toxicity are still poorly documented.

Distribution, Ecology, Chemistry and Toxicology of Plant Stinging Hairs

by Hans-Jürgen Ensikat 1,Hannah Wessely 2,Marianne Engeser 2OrcID andMaximilian Weigend1,*OrcID 1Nees-Institut für Biodiversität der Pflanzen, Universität Bonn, 53115 Bonn, Germany 2Kekulé-Institut für Organische Chemie und Biochemie, Universität Bonn, Gerhard-Domagk-Str. 1, 53129 Bonn, Germany

Toxins 2021, 13(2), 141; https://doi.org/10.3390/toxins13020141