The human immune system protects us from all kinds of invaders — from practically invisible viruses, to microscopic bacteria and protozoa, to large parasitic worms. Most of the time our immune system does a great job, attacking invaders and clearing them from our bodies. But sometimes it gets fooled and mistakenly attacks something benign, causing more trouble than benefit. An example of such a mistake is the poison oak rash.

Poison oak’s scientific name is Toxicodendron diversilobum, meaning toxic tree with diverse forms. Poison oak is a leafy plant that grows as both a bush in dense thickets and as a tree-climbing vine. It’s found throughout much of western North America and is common in the open country and hills surrounding the San Francisco Bay. Poison oak has myrtle green leaves in groupings of three in the spring that gradually turn reddish green then crimson over the course of the growing season, as it fruits a white berry. In the winter, only twigs and stems are left, making it especially difficult to identify.

Poison oak produces an oily resin called urushiol, which initiates the itchy rash. Studies show that it takes less than 50 micrograms, which is less than the weight of a grain of table salt, to initiate a rash in 90 percent of humans. Urushiol attaches easily to anything brushed against the leaves or stems of the plant. It can attach directly to skin or first to clothes and subsequently to skin. It’s not uncommon to get the rash from pets that have run through, or sports balls that have bounced into, poison oak bushes. The resin is tremendously stable. Dead plant specimens over 100 years old have been shown to cause the rash. If the plants are burned, for instance when clearing brush, the urushiol can get into the lungs, causing an especially dangerous reaction.

The rash occurs when some of the urushiol resin gets on and penetrates through the skin to cause a reaction called cell-mediated immunity. This is where our immune system makes the mistake. The molecules in the urushiol attach to the proteins and cells of our own body. This alters the appearance of these proteins and cells. Our own immune system is fooled by this altered appearance into thinking that a foreign invader, for instance a parasitic worm, is trying to enter our body. Our immune system marshals its diverse forces, releasing toxins locally to try to kill or repel the invader. Unfortunately the toxins also damage body tissues, causing the rash.

How can the rash be avoided? The first and best defense is to know how to recognize the plant and avoid it. If you need to go into an area with these plants, wear long pants and a long-sleeved shirt, and carefully remove and wash these in hot soapy water immediately afterward. If your pet or sports equipment may have contacted poison oak, wear gloves and give them a bath. There is now an over-the-counter product called Ivy Block that, if applied prior to exposure, physically blocks urushiol from attaching to skin.

If you get the rash, treatments are directed at reducing the local irritation, reducing the itch or stopping the underlying immune system reaction that caused the rash. The local irritation is typically treated with lotions such as calamine, cool water baths with oatmeal or baking soda or cool compresses. The itch can be treated with over-the-counter antihistamines such as diphenhydramine (e.g., Bendryl). Steroid creams or ointments, such as over-the-counter cortisone or more potent prescription strength steroids, can be used to quiet the immune response reaction. In severe cases systemic steroids may be necessary, either in pill form or, rarely, intravenously.

Poison oak is a fact of life in California, especially in the Bay Area. It’s best avoided, but can be an irritating reminder of just how hard our immune systems work to protect us from invaders.

— Bill Black, M.D., Ph.D.

— The Palo Alto Medical Foundation and column editor Arian Dasmalchi provide this monthly column.