The body's immune system normally protects us from bacteria, viruses, and other foreign invaders that can make us sick. urushiol which is from the poison ivy plantcauses many problems. When this touches the skin, it starts an immune response, called dermatitis. Dermatitis normally would otherwise be a harmless substance howevedr with poison ivy it differs.
Urushiol makes its way down through the skin, where it is metabolized, or broken down. Immune cells called T lymphocytes recognize the urushiol as a foreign substance,and or antigen. They send out inflammatory signals called cytokines, which bring in white blood cells. these white blood cells turn into macrophages to eliminate the substance. The macrophages eat foreign substances, but unfortunately when doing this they also damage normal tissue., when that happens it results in the skin inflammation to occurs with poison ivy.
The allergic reaction to poison ivy is known as delayed hypersensitivity. Unlike immediate hypersensitivity, which causes an allergic reaction within minutes of exposure to an antigen, delayed hypersensitivity reactions don't emerge for several hours or even days after the exposure.
Most people don't have a reaction the first time they touch poison ivy, but develop an allergic reaction after repeated exposure. Everyone has a different sensitivity, and therefore a slightly different reaction, to poison ivy. Sensitivity usually decreases with age and with repeat exposures to the plant. urushiol is found in all parts of the poison ivy plant the leaves, stems, and roots it's best to avoid the plant entirely to prevent a rash.