The mango that thought it was poison ivy
A Houston-area man had been intrigued by the taste of mango smoothies, a pleasing drink he encountered on several recent visits to a friends house. So when mangos went on sale at the local grocery store, this 27-year-old Texan decided to buy one.
To his surprise, the experience left a lasting impressionquite literallyas the photo, here, depicts.
The tropical import has a thick green skin. To remove it, the man firmly grasped the fruit in his left hand, using his right to peel it. When the phone rang, the man sat down to answer itwithout first washing his hands.
He was clad in shorts and at some point, while distracted by the phone conversation, rested his left hand on the inside of his right leg. Three days later, an oozing, blistery rash broke out.
While it resembled poison ivysomething he had experienced plenty of times during adolescencethe man could not fathom where he might have encountered the plant. So he showed the irritation to Mark O. Tucker and Chad R. Swan, residents in general surgery at St. Joseph Hospital, in Houston. They helped him to piece the puzzle together.
Hives, skin rashes, and other symptoms of allergy can occur upon repeated exposure to a chemical, usually a protein. People allergic to milk, for example, dont react the first time they encounter the beverage. A rash or other symptoms develop only after two or more exposures to it. Between those exposures, the immune system will have begun arming itself against the triggering chemical by manufacturing antibodies. Once those antibodies begin circulating, the body is primed to respond with a full-blown allergic reaction.
In some instances, however, the chemical responsible for triggering an allergic reaction may closely resemble those in other sources. And that, Tucker says, appears to be what happened here. Having been sensitized to poison ivy, the mans immune system was primed to respond to the plants rash-inducing oleoresin. It apparently encountered that resin, or one nearly identical to it, in the mangos skin.
This instant allergy to a new substance, after an earlier antibody-inducing sensitivity to another, is known as "cross reactivity." Dermatologists had previously observed mango cross-reactivity in persons sensitized to poison ivy. "What really surprised me here," Tucker told Science News Online, "was how dramatic its presentation was. It was almost like youd inked someones hand and transferred that print to the mans leg, because you could definitely see the shape of a hand and three fingers."
If the mans skin had previously been exposed to mango, the physicians would not have had to look for cross-reactivity to explain the hand print. However, Tucker points out, the smoothies that the man had so enjoyed had been prepared by others and consumed through a straw. So he should not have had any skin contact with oleoresins in the fruits skin or even with the drink.
An initial report of this distinctive rash appears in the July 23 New England Journal of Medicine.
Other examples of cross-reactivity
Cross-reactivity can be a particularly vexing problem because it allows novel allergens, without warning, to elicit full-blown allergic symptoms.
The problem frequently occurs among persons sensitive to peanuts, the most common food allergy. Though peanuts are legumes, they often foster a related sensitivity to tree nuts, such as almonds, filberts, and cashews. Similarly, individuals who become allergic to a fish, such as cod, may develop a cross-reactivity to other species, such as mackerel, herring, and plaice.
As with the mango and poison ivy example, however, potential sources of cross-reactive triggering agents can be far less obvious. Last year, for instance, a team of researchers from Bombay, India, published the first reports of persons with life-threatening allergies to fenugreek, a common spice derived from the seeds of a legume. Both affected women had a known allergy to chickpeas, another legume. Their coming from a related family of plants, the researchers said, suggests "[a] possible cross-reactivity between these two legumes."
In another instance, a 24-year-old Australian woman with an allergy to alcoholic beverages found herself hospitalized with wheezing, swelling of the lips and mouth, and a near loss of consciousness after eating rock melon. Despite having developed classic symptoms of anaphylactic shock, subsequent tests uncovered no evidence that the woman was actually allergic to the fruit.
However, in probing further, her doctors learned that the particular melon that had evoked the life-threatening symptoms had tasted overripe and a bit pungent. They concluded that their patients melon had begun to fermentproducing alcohol. In the March 1997 Annals of Allergy, Asthma, and Immunology, Dominic F.J. Mallon and Connie H. Katelaris, describe this as the first instance of alcohol-induced anaphylaxis stemming from overly ripe fruit.
Then theres the case of the 36-year-old woman living in Madrid who developed runny eyes and wheezingevidence of allergic asthmawhenever she touched leaves of the weeping ficus (Ficus benjamina) plants in her home. These small ornamental indoor trees have become an increasingly popular decoration in homes and offices.
Earlier this year, a team of immunologists reported that on three separate occasions, the womans tongue and throat started to itch and swell up after eating: a fresh fig triggered one episode, a dried fig provoked a second, and kiwi fruit induced the third. María Luz Díez-Gómez and her colleagues at Madrids Hospital Ramón y Cajal note that the fig plant (Ficus carica) is not closely related to the ornamental figs that flanked the womans living room sofa for 12 years. However, the physicians did find evidence that the woman exhibited allergic responses to several enzymes present in the fruitswhich may be a possible clue to their cross-reactivity with the ornamental ficus.
Still other reports have highlighted instances in which persons with allergies to latex rubber subsequently developed a cross-reactivity to fruit (such as bananas) and individuals allergic to birch pollen exhibited an apparent cross-sensitivity to kiwi fruit.
In recent years, a number of immunologists have begun campaigning for thorough labeling of ingredients in commercially prepared foods. The goal is to help persons with allergies identify products with the potential to trigger life-threatening reactions. However, these tougher labeling regulations would not shield people from exposures to all the host of compounds that may mimic their known allergens.
Díez-Gómez, M.L., et al. 1998. Asthma caused by Ficus benjamina latex: evidence of cross-reactivity with fig fruit and papain. Annals of Allergy, Asthma, & Immunology 80(January):24.
Hansen, Tine K., et al. 1997. Codfish allergy in adults: IgE cross-reactivity among fish species. Annals of Allergy, Asthma, & Immunology 78(February):187.
Mallon, D.F.J., and C.H. Katelaris. 1997. Ethanol-induced anaphylaxis following ingestion of overripe rock melon, Cucumis melo. Annals of Allergy, Asthma, & Immunology 78(March):285.
Patil, S.P., P.V. Niphadkar, and M.M. Bapat. 1997. Allergy to fenugreek (Trigonella foenum graecum). Annals of Allergy, Asthma, & Immunology 78(March):297.
Raloff, J. 1996. Peanut allergy found common and increasing. Science News 150(Sept. 7):150.
_____. 1996. Family allergies? Keep nuts away from baby. Science News 149(May 4):279.
Tucker, M.O., and C.R. Swan. 1998. The mango-poison ivy connection. New England Journal of Medicine 339(July 23):235.
Sampson, H.A. 1992. Food hypersensitivity: Manifestations, diagnosis, and natural history. Food Technology (May):141.
María Luz Díez-Gómez
Servicio de Alergia
Hospital Ramón y Cajal
Ctra.Colmenar Km 9.1
Dominic F.J. Mallon
c/o MacFarlane Burnet Centre for Medical Research
Yarra Bend Road
Fairfield, Victoria 3078
Pramod V. Niphadkar
Sir Hurkisondas Nurrotumdas Medical Research Society
Raja Rammohan Roy Road
Bombay 400 004
Mark O. Tucker
St. Joseph Hospital
Houston, TX 77006
This week's Food for Thought has been prepared by Janet Raloff, senior editor of Science News.
Food for Thought Archives
copyright 1998 ScienceService