Poison Ivy: an Exaggerated Immune Response to Nothing Much


Click on the links in this overview for more detailed information.

The poison ivy plant and its relatives are common throughout the United States. Poison ivy leaves are coated with a mixture of chemicals called urushiol. When people get urushiol on their skin, it causes an allergic contact dermatitis. This is a T cell-mediated immune response, also called delayed hypersensitivity, in which the body's immune system recognizes as foreign, and attacks, the complex of urushiol-derivatives with skin proteins. The irony is that urushiol, in the absence of the immune attack, would be harmless. The most common treatment for severe contact dermatitis is with corticosteroids, which diminish the immune attack and resulting inflammation. A recent recommendation for mild cases is to use manganese sulfate solution to reduce the itching. Jewel weed is also recommended.

Details on topics referred to in the above overview.

Allergic Contact Dermatitis

Allergy is an altered or unwanted immune response. The immune system attacks something which is genuinely foreign, but which would be harmless, were it not for the immune attack. The immune system has evolved to neutralize and eliminate foreign substances from our bodies. However, it cannot tell whether the foreign substance is harmful, so it sometimes attacks harmless substances vigorously, causing an inflammation which can be far more harmful than the foreign substance alone. Hay fever (allergic rhinitis or asthma to plant pollens) is a good example.

Dermatitis is an inflammation of the skin. If the allergy which causes the dermatitis is a response to something which came into contact with the skin, it is called allergic contact dermatitis. In addition to poison ivy, other things which contact the skin such as clothing, shampoo, jewelry, make-up, and deodorants can also cause allergic contact dermatitis. Allergic dermatitis can also be caused from within, as when a skin rash develops because of something we ate.

An extensive list of substances causing allergic contact dermatitis has been provided by Truett.


Cortosteroids are natural hormones in the body. They are immunosuppressive and antiinflammatory. They work by affecting gene expression in a complex manner, and the ways they reduce immune inflammation are not fully understood. They reduce the production of inflammatory cytokines and arachadonic acid and its derivatives (prostaglandins, prostacyclin, thromboxanes, and leukotrienes). The major natural antiinflammatory corticosteroid in the body is hydrocortisone (also called cortisol). Longer-lasting and more potent synthetic analogs of hydrocortisone are often used in the clinic, such as prednisolone, prednisone, or dexamethasone.

Delayed Hypersensitivity

Allergic inflammations, also called hypersensitivities, are caused by three underlying mechanisms. The one which is responsible for the reaction to poison ivy is called delayed hypersensitivity.

Although delayed hypersensitivity is responsible for the reaction to poison ivy, here, for comparison, is a brief introduction to the other two mechansims of hypersensitivity.

Poison Ivy Plant

The poison ivy plant is Toxicodendron radicans in the eastern US, and T. rydbergii in the midwestern US. T. radicans is also called Rhus toxicodendron or Rhus radicans. See
Truett and Nelson for more information. Western poison oak is Rhus diversiloba, and poison sumac is Rhus vernix.


Urushiol is a mixture of catchol derivatives. The major catechol on poison ivy leaves is pentadecylcatechol. If urushiol is washed off the skin within an hour or so, the reaction can be largely prevented. However, if left on the skin, some diffuses through the skin, where it is metabolized to
quinone derivatives. These form covalent complexes with skin proteins such as keratin. These complexes appear foreign to the immune system, which therefore attacks them.

Further information on poison ivy.

See Case studies in immunology - a clinical companion by Fred S. Rosen and Raif S. Geha, Garland Publishing Inc., 1996 (Case 6, contact hypersensitivity). Further information on allergy and hypersensitivity, and the underlying mechanisms, can be found in any
textbook of immunology. For a discussion of corticosteroids, see a physiology book, such as the current edition of the Review of Medical Physiology by William F. Ganong, Appleton and Lange.

Godfrey, H. P., H. Baer, and R. C. Watkins. 1971. Delayed hypersensitivity to catechols. V. Absorption and distribution of substances related to poison ivy extracts and their relation to the induction of sensitization and tolerance. J. Immunol. 106(1):91-102.

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Last updated March 31, 1997.