The α-methylene-γ-butyrolactones are part of a large and varied group of bioactive natural products widely present in plants. Their high chemical reactivity arises because of an α,β-unsaturated carbonyl system which allows the formation of covalent adducts with nucleophilic residues on diverse biomolecules. Most of these chemicals are of the sesquiterpene type (15-carbon-atom molecule) with an α-methylene group directed exocyclically to the γ-lactone ring. They are classified into 6 main groups of sesquiterpene lactone (SQL) structures: guaianolide, pseudoguaianolide, xanthonolide, ermophilanolide, eudesmanolide, and germacranolide (Fig. 1). They are the predominant allergens in the Compositae family as well as in other related plant families such as Jubulaceae, Magnoliaceae, Winteraceae, and Lauraceae.1 The α-methylene-γ-butyrolactones can also be found as more simple structures in the Liliaceae (tulipaline A) and Alstroemeraceae families.

Figure 1.

Chemical structures of the main groups of sesquiterpene lactones.

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The Asteraceae/Compositae family—also referred to as the aster, daisy, or sunflower family—comprises the second largest family of flowering plants in the world. It includes more than 22750 currently accepted species, spread across 1620 genera and 12 subfamilies. Over 200 species are important causes of contact dermatitis worldwide.2,3 This family presents a variety of edible plants, such as lettuce, chicory, dandelion, salsify, sunflower, scorzonera, artichoke and yacon. Many members are decorative flowers, such as chrysanthemums, gerbera, calendula, dendranthema, dahlias, and heleniums while many others are considered as common weeds. Species such as arnica, chamomile or feverfew are also used medicinally. Asteraceae/Compositae can be recognized by its inflorescence, consisting of many tiny flowers clustered to form a flower head and subtended by an involucre.

SQLs are the predominant allergens although other compounds such as epoxythymol-diesters and polyacetylenes may also be involved.4 SQLs are usually lipophilic, and are often located in the liquid-soluble part (oleoresin fraction) of the leaf, stem flower, and possibly in polen.2,4,5 Although the mechanism of sensitization is unknown, several possibilities have been proposed: pollen or dry debris airborne transportation, direct and/or indirect contact with parts of the plant, and allergen inhalation or ingestion.4,6

Some peculiar cutaneous eruptions caused by the Asteraceae/Compositae family have been described. The pseudophotodermatitis pattern occurs almost exclusively in middle-aged men with a history of outdoor exposure but is rare in children and women, although some studies have suggested an equal sex distribution.6,7 Exposed areas of the face, V of the neck, hands, and forearms are typically involved. In addition, areas not clearly exposed to sunlight such as retroauricular regions (Wilkinson triangle), eyelids, nasolabial folds, and the area beneath the chin are also involved, allowing its differentiation from a true photo-related dermatitis.7 In hot regions, during summer months dry dead plant material contributes to this airborne pattern of dermatitis. In the US, many Compositae weeds, including ragweed (Ambrosia species), induce this clinical pattern often called ragweed dermatitis8 or weed dermatitis in regions where other composite weeds predominate, such as Ambrosia, Artemisia, Helenium, and Iva species.9,10 A similar pattern was described in Australia where it is known as bush dermatitis because it is caused by species such as Arctotheca, Cassini, Conyza, Cynara, and Dittrichi11,12 and also in India where it is called parthenium dermatitis, where the culprit plant is Parthenium hysterophorus L13,14 Although these environmental conditions are not normally encountered in the temperate regions of Europe, some isolated reports of airborne dermatitis (ragweed dermatitis) induced by feverfew (Tanacetum parthenium [L] Sch. Bip.), chicory and lettuce, liverworts of the genus Frullania, and chrysanthemums1,6,9 have exceptionally been reported.

A second special clinical pattern has been described as atopic eczema-like eruption. Compositae allergy may be manifested as a generalized eczematous eruption with primarily flexural involvement. Although it most commonly occurs in elderly men in rural areas, cases in children have also been reported.4,15 An erythrodermic exfoliative pattern classically induced by the weed Parthenium hysterophorus L was also described. This plant was accidentally introduced to the west coast of India in contaminated seed wheat. Several thousands of cases of allergic contact dermatitis occurred, some of them fatal.16

Hand eczema is more frequent in women working in gardening after contact with weeds. A palmar distribution often predominates.17 Hand dermatitis is also associated with handling lettuce.18,19 Dermatitis confined to one or more localized areas such as facial dermatitis secondary to steaming chamomile tea or hand and arm dermatitis from herbal compresses20 has rarely been reported. Oral swelling, perianal pruritus and dermatitis of the trunk and arms were described in a sensitized patient who drank chamomile tea.20 Stomatitis and throat swelling have occurred in some patients after eating lettuce and endivia.21 In one single patient erythema multiforme was reported to develop after patch testing.22

A rare chronic and disabling eccematous photodermatosis known as chronic actinic dermatitis (CAD), photosensitive dermatitis, or actinic reticuloid, has been linked to the ACD to compositae. A high incidence of ACD to one or more allergens has been found among patients with CAD,23 and sesquiterpene lactones are commonly identified as allergens. It is thought that about 20% to 75% of patients sensitized to Asteracea (Compositae) have some degree of photosensitivity. Thus, the progression of ACD to sesquiterpene lactones to CAD has been postulated, but its origin is still unknown.

Patch Test

No ideal patch testing material to study Compositae-induced dermatitis exists. The Compositae family includes more than 1350 SQLs and almost 50% of them are potential contact allergens.2 SQLs with slight differences in molecular structure, may cause true cross-reactions, while identical SQLs from different species may be responsible of false cross-reactions. SQLs are not exclusive to the Compositae family and frequently show cross-reactivity with other plant families. Because the concentration of SQLs shows seasonal and geographic variations and as cross-reactions between different species are not complete, when a patient is not tested with the relevant plants, the diagnosis may be easily overlooked.

Fresh plants or plant extracts should not be used for patch testing because they may cause severe contact reactions and can also elicit active sensitization. False positive irritant reactions or false negative results caused by low content of allergens (degradation products) can also occur.

In 1986, Benezra and Epstein developed the sesquiterpene lactone mix (SL mix) a nonsensitizing and seldom irritant mixture of 3 of the 6 main structural groups of sesquiterpene lactones (alantolactone, costunolide and dehydrocostuslactone of eremophilanolide, germacranolide, and guaianolide group) (Fig. 2). This mixture can enable identification of about 60% of SQL sensitization cases24 and was included in the European baseline series.17 Other studies19,25–27 reported different percentages, which can partly be explained by phytogeographical variations and the diversity of SQL structures.28 An alternative preparation, the Compositae mix, based on a mixture of plant extracts (arnica, yarrow, tansy, German chamomile, and feverfew), detects a higher proportion of the cases; however, it may be irritant and cause patch test sensitization.29

Figure 2.

Structures of alantolactone, dehydrocostuslactone, and costunolide, constituents of the SL mix test material.

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Dandelion and feverfew extracts, together or individually, also appear to be more useful than the SL mix.30,31 Other blends have been proposed, such as a mixture of Achillea millefolium L, Chamaemelum nobile All, Helianthus anuus L, Tagetes minuta L, and Tanacetum vulgare L as well as extracts mixtures of arnica, German chamomile, feverfew, tansy, and yarrow.27

Jubulaceae Family (Liverworts)

Liverworts are nonvascular plants with a bryophyte life cycle. They comprise 4 genera: Frullania accounts for nearly 800 species and Jubula for 15, whereas Steerea and Neohattoria are both montypic. These plants are often epiphytes growing on trunks and branches of trees or shrubs. They are found throughout the world and are common in tropical and subtropical regions but rare in frigid climates. They are often resistant to desiccation and direct light. Jubula species are largely confined to damp shaded rock surfaces.

At least 11 species of Frullania have been described as responsible for allergic contact dermatitis.32Frullania dilatata (L) Dum and Frulanina tamarisci (L) Dum have been frequently implicated in Europe. Cases of allergic contact dermatitis were also described in British Columbia (Canada), the Pacific North-West of the United States, and in the South-West of France.

The most important allergen is frullanolide,33–35 an α-methylene-γ-butyrolactone sesquiterpene from the eudesmanolide structural group. Other α-methylene-γ-butyrolactones have been reported as sensitizers such as oxyfrullanolide 5, cis-b-cycloscotunolide 6, and eremofrullanolide 7.35–37Frullania dilatata (L) Dum contains exclusively the d- or (+)-enantiomer of frullanolide while Frullania tamarisci (L) Dum exclusively yields the l- or (−)-enantiomer.36 Both enantiomers are sensitizers, and in guinea pigs the stereospecificity of sensitization to one enantiomer has been demonstrated36,37 (Fig. 3).

Figure 3.

Chemical structures of d-(+)- and l-(−)-frullanolide.

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Sensitization to Frullania can occur by direct handling of trees or by airborne contact through volatilization of its small particles, especially during warm weather or by the action of chainsaws.34,35 Forest workers often suffer from woodcutter's eczema and domestic allergy can present in individuals who use lobe-leaved trees as firewood. Exposed areas of skin are typically affected, with an airborne pattern on the face and the V of the neck. This clinical pattern can mimic a photosensitivity pattern but it involves areas with little photoexposure such as the eyelids and nasolabial folds. The hands may become edematous, with involvement of finger webs but often sparing of the palms. Exceptional involvement of the genitalia and intertriginous folds has also been reported.38

Patch Test

Testing the patient with small pieces of Frullania is associated with a high risk of active sensitization. The SQL mix only detects about 1% of allergic reactions to Frullania. The frullanolide mix, a racemic mixture of (+)- and (−)-frullanolide at 0.01%, 0.033% and 0.1% in petrolatum39 is a safe and nonsensitizing test, but only shows 0.35% of positive reactions.

Although testing the patient with small pieces of Frullania is associated with a high risk of active sensitization, it could be used if there is high clinical suspicion. Frullanolide mix, a racemic mixture of (+)- and (−)-frullanolide at 0.01%, 0.033% and 0.1% in petrolatum39 has been used in several clinical trials proving to be a safe and nonsensitizing test, but with a lower sensitivity than the SQL mix. Therefore the allergen of choice in these cases is SQL mix, even though it only detects 1% of the allergic reactions to Frullania.

Cross-reactions with botanically unrelated plants that contain similar allergens, eg Laurus nobilis L,35Magnolia grandiflora L,40 and several Compositae may also occur.32,41,42

Liliaceae

The tulip genus which comprises 109 species of showy flowers is the most important genus of the Liliaceae family. A number of species and many hybrid cultivars are grown in gardens, used as pot plants, or displayed as freshly cut flowers. Most cultivars of tulip are derived from Tulipa gesneriana. Professional bulb growers, bulb collectors, sorters, and packers are particularly at risk of contact dermatitis.

Tulip bulbs contain several glucosides including tuliposides A and B, which are weakly allergenic but are rapidly hydrolysed to tulipalin A and B. Tulipalin A appears to be the main sensitizer in clinical practice43 although tulipalin B also causes sensitization in guinea pigs44 (Fig. 4). Flowers, leaves and stems possess higher concentrations of tuliposide A than the bulbs; but sensitivity can develop from contact with any portion of the plant, although most of the cases occur in bulb growers who are continuously handling the bulbs. Contact with airborne allergens, although infrequent has also been reported.45

Figure 4.

Chemical structures of tulipalin A and B.

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The most common presentation is a combined allergic and irritant hand dermatitis known as tulip fingers. It is characterized by painful, subungual and periungual erythematosquamous, sometimes erosive lesions involving the fingertips, particularly the first and second fingers of the dominant hand.46 There may be fissures, suppuration, and hyperkeratosis. The dermatitis may extend to the adjacent skin and rarely may involve distant areas or even give rise to a diffuse dermatitis.45,47 The dust in tulip sheds may induce conjunctivitis, rhinitis and asthma. Reaginic responses to tulip antigens can also occur.47

Alstroemeriaceae

These species have a cluster of tuberous roots and produce leafy stems, bearing one to several brightly colored flowers. Several species are found in special collections and only 2 species are widely distributed and produced: Alstroemeria aurantiaca D Don, with orange flowers, which grows as a border plant, particularly in cottage gardens, and Alstroemeria ligtu (Peruvian lilies) which give rise to numerous colors, mostly in shades of lilac and pink, with darker veins and mottling on the petals (Fig. 5). They grow in Holland, South America, USA and Australia. Tulipalin A, the same allergen of the Liliaceae family, is the main responsible sensitizer and is found mainly in the petals and stem. The rate of sensitization for tulipalin A can exceed 50% in workers on Alstroemeria plantations.48

Figure 5.

Alstroemeriaceae.

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Dermatitis most commonly affects florist who strip off the leaves manually and are therefore exposed to the broken plant with a high risk of allergic contact dermatitis. The lesions, in contrast to tulip finger dermatitis, can be more severe and tend to involve the fingertips of all digits of both hands. Occasionally, the clinical pattern can be very similar to that produced by the garlic clove (but in a bilateral and symmetric distribution).49 Workers who cut the flowers often show palm eczema.50–55 Airborne dermatitis56 and post-inflammatory depigmentation have rarely been reported.57 False positive responses due to a primary irritant reactions may also occur.58

This is a group of mono- or dihydroxybenzene derivatives of phenol, catechol, resorcinol, or salicylic acid with a long alkyl or alkenyl carbon side chain. Mixtures of catechols with alk(en)yl side chains are also known as urushiol and are the main allergens in the toxicodendron genus. Phenols, resorcinols, and salicylic acid with alk(en)yl side chains are called cardanol, grevillol, and ginkgolic acid, respectively and are the main allergic contact sensitizers in the Anacardiaceae, Gingkgoaceae and Proteaceae family respectively. They are very good examples of prohaptens, that is, nonelectrophilic substances which have to undergo a chemical transformation such as oxidation to yield the active substance. Thus, many phenolic derivatives are metabolized into ortho-quinones or para-quinones, which are strong electrophiles62 (Fig. 6).

Figure 6.

General structures of sensitizing alk(en)yl aromatic derivatives.

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The Anacardiaceae family also known as the cashew family or the sumac family is a very broad family of flowering plants. This family includes about 70 genera and 650 species of evergreen deciduous trees, shrubs, and woody vines. These plants are native to tropical and subtropical areas of the world, but some species are also found in temperate regions. Members of the Anacardiaceae plant family that are important in dermatology include: the Toxicoendron genus, (common poison ivy, poison oak, poison sumac), Mangifera indica L (mango), Toxicodendron vernicifluum (lacquer tree), and Anacardium occidentale L (cashew nut).

Toxicodendron is the most allergenic genus of this family. It includes poison ivy (Toxicodendron radicans Kuntze and subspecies such as T radicans Kuntze var, rydbergii Erskine), poison oak (Toxicodendron diversilobum Greene and Toxicodendron toxicarium Gillis), and poison sumac (Toxicodendron striatum Kuntze, T vernix Kuntze) complexes. These genera are the main causes of allergic contact dermatitis in the United States and approximately 50%–70% of the population seems to be sensitized.63

Occupational contact dermatitis to poison ivy is an important cause of occupational disability and represents a significant cost for the health system in the United States. Wilderness, rural and outdoor activities as well as outdoor occupations related to agriculture, forestry, and fire fighting, bring with them a high risk of sensitization.64

Poison ivy dermatitis is generally unknown in Europe. The plants can be recognized by their compound leaves, with 3 or more smaller leaflets. Flowers and fruit arise in the angle between the leaf and the twig. The leaf stalk also leaves a “U”- or “V”-shaped scar after it falls off.65

The pentadecylcatechols, consisting of 1, 2-dihydroxy benzenes with a 15-atom side chain at position 3 (3-pentadecylcatechol), also called urushiols, are the sensitizers in Toxicodendron plants. Urushiols are colorless or slightly64 yellow in their natural state, but they oxidize, polymerize and turn black when exposed to air. Urushiol is found in the stems, roots, leaves, and skin of the fruits of these plants and also on fomites, where it retains its antigenic potential in the dry state indefinitely.66 Young leaves contain greater amounts of urushiol than the older ones.67

Contact dermatitis primarily results from direct contact with the oleoresin obtained from a portion of a bruised or injured plant. However, contamination and re-exposure to the antigen may occur, with the most important reservoirs being the fingernails, clothing, animals, garden tools, or firewood.10,68 Contact dermatitis usually appears within 2–4 days; however it may appear as late as 2 weeks after the moment of contact. It is characterized by intense pruritus and an erythematous reaction followed by the appearance of multiple papules and vesiculo-bullous lesions in a linear distribution (Fig. 7). The face, neck, and genitalia are commonly affected and usually show intense edema. The dermatitis has a self-limiting course, lasting approximately 1–2 weeks, but it can last up to 6 weeks.64

Figure 7.

Erythematous linear arrays and erythema multiforme-like lesions on the right elbow after contact with poison ivy.

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Other clinical presentations include erythema multiforme-like reactions,69 exanthematous and urticarial eruptions, and even renal damage that may occur after accidental, medicinal or alimentary ingestion.70,71 Stomatitis and anal itch have been described after chewing the leaves or secondary to hyposensitization treatments. Black spot poison ivy dermatitis is a rare and very special condition, consisting of asymptomatic black lesions on the skin that cannot be washed off. These lesions are followed by subsequent pruritic papules and have been mistaken for melanoma in several cases.72

Ginkgo biloba L, the ginkgo tree, is the only representative of this family and is regarded as one of the world's oldest surviving tree species. Originating in southeastern China 200 million years ago, it was introduced to Europe during the early 18th century, and it now grows in temperate regions throughout the world. Contact dermatitis from the ginkgo tree is not due to its leaves but to its malodorous fruit exclusively borne in female trees.

The alkylphenols (anacardic or ginkgolic acids, cardols, and cardanols) have been identified as the potential allergens. Ginkgolic acid 1, the most important allergen, is found mainly in the fruit, but it can also be detected in Ginkgo leaves.89 Allergic reactions seem to occur only with open fruit extruding pulp. Handling the intact fruit is not harmful to sensitized persons.90

Contact dermatitis occurs especially in East Asians, who enjoy eating the nuts (seed kernels), and manipulate the seeds to remove the fleshy pulp and obtain the kernels. Pruritic, edematous and erythematous plaques with papules or vesicles appear on the hands, face, arms, and anywhere the seed pulp is spread. Ingestion may also cause stomatitis and anal pruritus.91 While reactions are uncommon in the West, a small epidemic occurred among a group of schoolgirls who stomped upon fallen fruit. They developed a streaky, vesicular dermatitis on their legs, similar to that of poison ivy contact dermatitis.92

The Proteaceae family comprises 1050 species in 62 genera and is found predominantly in the tropics. The most important genus of this family is the genus Grevillea which is made up of approximately 360 species, most native to Australia, where they range from low shrubs to forest trees. A number of different Grevillea species and hybrids, especially of the eugrevillea section, appear to cause allergic contact dermatitis.94 The most important species are Grevillea robusta Cunn, also known as silky oak or silver oak and Grevillea x Robyn Gordon, a natural hybrid between Grevillea banksii R Br and Grevillea bipinnatifida R Br which is perhaps the most important plant in this genus from a human health perspective and probably represents the most common cause of plant-induced dermatitis in Australia.95

The allergens appear to differ between the species and their hybrids. The allergens of G robusta are 5-tridecylresorcinol (grevillol), 5-pentadecylresorcinol and 5-pentadecenylyresorcinol. Grevillea banksii R Br shows the resorcinol derivative pentadecylresorcinol. Other long carbon chain alkenyl resorcinols include the haptens in Grevillea Robyn Gordon.96

Contact with the wild and cultivated tree, and especially with the flowers, as well as with objects made with its wood, induces a bullous eruption similar to that caused by poison ivy and poison oak (Toxicodendron sp).97 Grevillea poisoning was described among men who cut the fallen trees and developed a severe bullous contact reaction.98

Primulaceae is a broad family of flowering plants with about 1000 species in 24 genera. Despite the large number of specimens and cosmopolitan distribution only Primula sp presents a common dermatological hazard. Primula was exported from China to England in the year 1880. Their beautiful flowers, long flowering period, and their ease of care led to a quick introduction into Europe although success was more limited in America. For a number of years Primula obconica Hanse was the most common cause of plant-induced contact dermatitis in Europe; however since the introduction of primin-free P obconica Hanse into the European market, its incidence has declined.99 Other species of Primula are reported to be allergenic, such as P auricula L, P denticulata Sm and P vulgaris Huds, and vary widely in their allergen content.100–102

The primin (2-methoxy-6-n-pentyl-p-benzoquinone) is the major allergen of primrose dermatitis, a quinone formed by oxidation of its biosynthetic precursor miconidin (Fig. 8); however other allergens such as primetin (a flavone),103 and miconidin (2-methoxy-6-pentyl-1,4-dihydroxybenzene)104 have also been suggested. Primin is contained in the terminal cells of the microscopic glandular hairs on the surface of the leaves, stems, and flowers. The allergen content of the plant varies considerably, depending on season, the number hours of sunshine, the method of cultivation, and the horticultural variety, which explains the striking seasonal variation in its allergenicity, greatest in summer and lower in the winter months.

Figure 8.

Chemical structures of primin and miconodin.

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Sensitization to P obconica Hance can occur by direct contact with plants or by indirect contact, e.g. via hand-shakes, door handles, or paper fibers. Primin can be released through pollen or small dust particles, causing an airborne allergic contact dermatitis in some sensitized patients.105

Dermatitis most commonly occurs in elderly women and a variety of cutaneous reactions have been described: facial dermatitis, alone or combined with limb dermatitis and hand dermatitis.42,106,107 Eruptions vary from linear to erythematous papules, vesicles and bullae, and to lesions resembling erythema-multiforme and lichen planus.108–113 A few cases of airborne dermatitis111,114,115 and facial eruption mimicking herpes simplex116 as well as one case presenting as vitiligo117 have also been reported.

In order to reduce the incidence of allergic primin contact dermatitis, a primin-free Primula obconica Hance was developed.118 Although its introduction reduced significantly the cases related to cultivars, it still remains as an important contact allergen, justifying its maintenance in the standard series.

This family includes 900 species in 150 genera which are found in tropical and temperate regions especially in southern Africa and Australia. It is an important cause of phototoxic reactions although some cases of ACD to the citrus genus have also been reported.

Twelve species are native to southern China, south-east Asia and Indo-Malaysia; however, with the aim of achieving larger fruits, with more juice, better taste and lack of seeds, numerous other species have emerged from various artificial crosses. The most common species are Citrus sinensis L (orange), Citrus aurantium L (Bergamot), and Citrus Limon L Burm. Fil. (lemon). Besides its use in the food industry, citrus is used for aromatic oils in the flowers and fur, in perfumery, and in the manufacture of soaps and cosmetics.

Allergic contact dermatitis to citrus fruits is unusual and hand dermatitis has been described in food handlers.119 The potential sensitizers include geraniol, citral and the hydroperoxide derivative of d-limonene,120 which are usually included in fragrances. The mechanism of sensitization is currently an object of research.

The Brassicacea family contains about 3200 species in 375 genera. It includes several important vegetables, such as cabbage (Brassica oleracea L), cauliflower (B oleracea L v botrytis), radish (Raphanus sativus L), horse radish (Armoracia rusticana P Gaertn, B Mey, and Scherb), turnip (Brassica campestris L) and mustrard (Brassica nigra [L] WDJ Koch). The Capparaceae, regarded as the tropical relative of the mustard family, comprises some 650 species of small trees and shrubs in 30 genera. The most important representative of this family is Capparis spinosa L, or caper bush, a shrub with grayish oval leaves and showy white-pink flowers with several prominent stamens. It is better known for the edible bud and fruit which are usually consumed pickled.

Several species contain thioglucosides, which are broken down enzymatically, in the presence of water, to isothiocyanates, potent irritants, which appear to be also the most important allergens. Contact dermatitis from these plants can be both irritant and allergic,136,137 with a high potential irritant capacity. Allergic contact dermatitis is rare.

Allergic contact dermatitis is more commonly seen in food handlers. Raphanus sativus L induced an acute vesiculo-bullous dermatitis of both palms in a waitress who chopped them.135 Cabbages provoked occupational contact dermatitis138 and cabbage juice can produce positive patch test reactions in patients with hand dermatitis suspected to have been caused by vegetables.139Brassica nigra induced an allergic contact dermatitis due to mustard powder in a salad maker137 and Brassica oleracea v. Botrytis (cauliflower) induced a hand eczema in a plant grower.140Capparis spinosa L induced an acute vesiculo-bullous allergic contact dermatitis in a woman who applied a wet compress of the minced leaves and fruits to her arm.141

Patch test: Isothiocyanates present in these plants are usually primary irritants, and they can be tested at a concentration of 0.05% in petrolatum. As the irritancy of mustard oil tends to overshadow its allergic potential, a positive patch test with mustard can be difficult to interpret.135

The polyacetylenes are a group of chemically reactive and biologically active compounds with a carbon-carbon triple bond or alkynyl functional group. They can be found in 7 families of plants, and the majority has been isolated from the closely related plant families Araliaceae and Apiaceae. The polyacetylenic derivatives of the Araliaceae and Apiaceae are mostly the aliphatic C17-polyacetylenes of the falcarinol-type, derived from oleic acid.142 They act as very reactive alkylating agents towards, for example, thiol groups in proteins, forming hapten-protein complexes (antigens).142,143

This family of some 700 species in 55 genera consists mostly of trees and shrubs, but includes some twiners. Most species occur in tropical regions, particularly in Indo-Malaysia and tropical America. Other species are native to temperate regions, and some species have become widely distributed by horticulture and as houseplants.

Perhaps the best known temperate species is Hedera helix L, common ivy. The plants are native to Europe, North Africa, and Asia but are now grown in many other countries. Three main subspecies are found in Europe, namely helix (English ivy), canariensis (Canary Island ivy), and poetarum. They are a family of evergreen climbing plants usually found scrambling over rocks and through hedges or climbing up trees or walls (Fig. 12).

Figure 12.

Hedera helix.

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Falcarinol (heptadeca-1,9-diene-4,6-diyne-3-ol) and didehydrofalcarinol are the most important allergens. They are detected in stalks, leaves, and roots and may cause both irritant and allergic contact dermatitis. Falcarinol seems to be the stronger sensitizer and concentration and the ratio didehydrofalcarinol:falcarinol seems to vary considerably, depending on climate and other conditions of growth.144Sensitization can occur through contact with plants on climbing trees covered by them or by pruning or handling the plants in an occupational setting. Any site in contact with the plant may be affected and dermatitis commonly begins with an itching and erythema that in a few days develops into a vesicular eczema (Fig. 13).145 The most frequently affected sites are hands and forearms followed by involvement of the face and neck; however, involvement of trunk and legs as well as widespread dermatitis has also been reported.145,146 Urticarial-like dermatitis has also been reported.147 Immediate, possibly allergic, reactions from common ivy causing rhinitis and asthma have rarely been reported, The majority of mucosal symptoms in gardeners are probably irritant, perhaps caused by hederin.148–150

Figure 13.

Symmetrical erythematous patches after contact with Hedera helix.

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Also known as Umbelliferae, this family of 2850 species in 275 genera is of cosmopolitan distribution but is found mainly in north temperate regions. Most of the species of dermatological importance are similar in floral structure, with cluster of flowers on stalks of roughly equal length arising from a single point as an umbrella structure. Most of the members of this family have been shown to be rich in psoralenes and therefore produced phototoxic reactions. Some ornamental plants and weeds as well as edible plants of this family have also been related to allergic contact dermatitis. Two important species in dermatology are Centella asiatica and Daucus carota vars.