Seabather's eruption

Seabather's eruption is an itching dermatitis[1] caused by a hypersensitivity reaction to the immature nematocysts of larval-stage thimble jellyfish (Linuche unguiculata), sea anemones (Edwardsiella lineata) and other larval cnidarians.

[citation needed] The reaction is identified by severe itching around small red papules 1mm to 1.5 cm in size on areas of skin that were covered by water-permeable clothing or hair during ocean swimming.

Initial swimmer exposure to the free-floating larvae produces no effects, as each organism possesses only a single undeveloped nematocyst which is inactive while suspended in seawater.

However, due to their microscopic size and sticky bodies, large concentrations of larvae can become trapped in minute gaps between skin and clothing or hair.

Once the swimmer leaves the ocean, the organisms stuck against the skin die, and automatically discharge their nematocysts when crushed, dried out, or exposed to freshwater.