Sometimes there are nice correlations between an organism’s defenses and its enemies, but in the case of poison ivy, any such connection is a puzzle.
Poison ivy is toxic to humans and a few higher primates and that’s it.
As close a relative as the rhesus monkey, for example, is normally not sensitive to it.
Fossil ancestors of poison ivy predate human settlement of North America by millions of years. It is almost as if nature anticipated that it would be a good thing to keep humans off beaches.
Poison ivy acts as a soil binder and grows especially well in very sandy areas.
It was even imported into the Netherlands to bind some of the dikes.
In late summer, its white berries are an important food for migrating birds. In fall, the leaves turn a brilliant salmon color when conditions are right, and it has attractive white seed heads that have lured some flower arrangers and given them a rash.
The chemical that produces the rash is an oily substance in the sap called urushiol.
It flows in special channels, and the leaves and stems must be bruised to release the oil. Contrary to folklore, merely being close to poison ivy is not dangerous unless the plant is burned, so that the volatile oil moves into the air.
When urushiol binds with skin layers, the body detects a foreign agent and sends white blood cells rushing to defend the area.
The white blood cells overreact and attack the skin cells, too. The whole area becomes inflamed as the white cells release cell-destroying substances. Where the oil is concentrated, blisters form.
Poison ivy belongs to the same family as cashews and mangoes.
The apple-like cashew fruit has nuts in a green shell hanging out from one end, and it is full of volatile irritating oils that must be roasted out.
Biting into a mango’s skin can also cause blisters around the mouth.