The urushiol oils in various parts of the poison ivy plant cause an allergic rash, a type of contact dermatitis. The thick oil attaches to clothing, skin, and pet fur. The rash that can develop from a run-in with poison ivy is not contagious itself; the blister sacs that form during a breakout do not contain urushiol. It is oil passing from one surface, or one person, to another that causes and prolongs a person's reaction to poison ivy. Many home remedies can treat this reaction, and the rash generally heals after two or three weeks without long-term complications.
Redness as a symptom is not a definitive indicator for poison ivy, as sunburn and other allergens, as well as many conditions, can cause this sign. The color change occurs when the body reacts to the irritant -- blood rushes to the skin's surface to begin fighting off the invader. Skin redness can become uncomfortable if the reaction persists for an extended period.
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