Consuming any part of poison ivy containing the chemical that causes irritation can cause a fatal reaction, so eating it should be avoided. Introduction of the irritant chemical this plant creates into the throat or stomach inflames everything along the way and can cause serious problems with breathing.
The chemical that causes the allergic reaction found in poison ivy, and many other plants that cause similar irritating redness and swelling, is called urushiol. This allergen occurs naturally as a means to repel creatures that would try to eat the plant, but some animals have developed an immunity to the poison through evolution. It is not possible to build up an immunity to urushiol through exposure. Some people may not display any allergic reaction to this chemical at all much like peanut allergies are only found in some people.
Urushiol is found in poison sumac, poison oak, poison ivy and the Lacquer tree. In all of these cases, it's transferred onto the skin when brushed against and causes a reaction well after contact with the plant. Lacquer trees are called that because of their sap, which contains enough urushiol to produce a hard lacquer with the right moisture conditions, making it a good choice for the creation of lacquerware.