The old adage “leaves of three, let it be” helps to identify and therefore avoid poison ivy. The plant is polymorphic, growing as ground cover, small shrubs up to two or three feet high, or a climbing vine. It is often confused with virginia creeper which has five leaves. Formally named Toxicodendron radicans, it and other related plants contain urushiol (oo-rush-ee-ol) . The substance is present in all parts of the plant; leaves, stems, roots, and berries. Poison ivy, poison oak, and poison sumac are known to cause the characteristic itchy, blistering rash.
Other plants such as cashews also have urushiol when raw and must be properly processed, shelled and roasted, to remove the plant oil. Lacquer, the stuff that produces the beautiful shiny finish in furniture, contains small amounts of urushiol and can affect hypersensitive individuals. The name urushiol comes from the Japanese name for Lacquer, urushi.
The first time, or first few times one is exposed there usually isn’t a reaction. Only after being exposed does one become sensitized and on second exposure develop the itchy rash. This is because urushiol is an allergen, something which causes an allergic reaction , and and the allergy has to be “learned” by exposure.
Actually it is a bit more complex. Only proteins, very large molecules, can cause an allergic reaction. Much of our bodies are protein so our immune system must be able to distinguish our proteins and foreign proteins. Antibodies develop as a method to rid the body of foreign protein. A whole cascade of chemical reactions occur once the immune system identifies a foreign protein. Urushiol is not a protein but a substance know as a hapten. Haptens are small molecules which can chemically react with protein. Once a protein, for example the keratin of our skin, has reacted reacted with urushiol our bodies no longer recognize the protein as “self” but rather as foreign.
Urushiol is a fat soluble oleoresin, which means that it can penetrate the skin within an hour or two. There it reacts, “labels” the protein, and sets the allergic reaction in motion, the rash occurring several hours after exposure. Heavier exposures result in a faster reaction.
This has lead to the misconception that the allergen may be carried through the blood. For example heavy exposure to the back of the hand and only slight exposure to the upper arm means that the rash will show up on the hand first and arm only later.
Another misconception is that the fluid present in the blisters contains the poison. Once the blisters form, the poison is long gone. You can’t get a reaction just by coming in contact with someone else’s rash. You can be exposed by secondary contact however. There is a fairly wide variation is sensitivity so exposure and subsequent reaction can occur when a sensitive person handles the clothes of a less sensitive person.
In extreme cases, people are exposed by inhaling smoke particulates from burning poison ivy. This can be dangerous as the rash occurs in the throat and even in the lungs.