Stinging hairs on the stem of a stinging nettle, Urtica dioica. The sting comes from the longer hairs (the short ones are harmless); these have silicified tips which break off when the nettle is brushed against or stroked (not grasped). The sting penetrates the skin, injecting a mixture of acetylcholine & histamine; the former is a neurotransmitter substance normally produced at nerve terminals, the latter a chemical mediator normally produced by mast cells & responsible for many of the unpleasant symptoms of allergic reactions.
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