This family of some 600 species in 30 genera is widely distributed throughout the world, but not in Australasia. The family is taxonomically intermediate between the Liliaceae and the Amaryllidaceae, and was formerly included in the Amaryllidaceae.
The largest and most important genus is Allium L., the 450 species of which are widely distributed in the Northern Hemisphere. Some of the strongly flavoured species have been culinary and medicinal items since early times (Numbers). Garlic (Allium sativum L.) was an important part of the diet of the Egyptian slaves who built the Pyramids, and also of the legions of ancient Rome (Collins 1964).
The onion (Allium cepa L.), chives (Allium schoenoprasum L.), shallot ("Allium ascalonicum"), and leek (Allium porrum L.) are very widely grown. Several other species are cultivated for culinary use, and others for their attractive flowers. Some, such as Allium triquetrum L., have become abundant and troublesome weeds.
Some species, particularly Allium sativum L., find limited use in medicine.
Members of the genus Allium have often been reported to have irritant and allergenic properties on skin contact. Although farmers and gardeners may frequently be exposed to Allium species, the most intimate and prolonged contact is liable to occur in cooks, housewives, and other food handlers. The chemicals responsible appear to be the volatile sulfur-containing oils that are released when the plant material is damaged. The more familiar lachrymatory effect that is experienced when peeling or chopping the onion (Allium cepa L.) is caused by the release of thiopropanal S-oxide.
Boscher et al. (1996) reported the presence of S-2-propenyl-L-cysteine sulfoxide in quantities similar to those found in cultivated garlic (Allium sativum L. — see below) in the bulbs of this cultivated variety found on the Ile d'Yeu in France.
The botanical name Allium cepa now refers to a diverse collection of cultigens that can be segregated into three groups, namely the Cepa, Aggregatum, and Proliferum groups. The Cepa group comprises the various single-bulb onions that do not produce bulbils in their inflorescences; the Aggregatum group comprises the shallots, which form a number of lateral bulbs and no bulbils in their inflorescences; the Proliferum group, comprising the Egyptian or tree onions, is characterised by the formation of bulbils in their inflorescences from which the plants may be propagated. The Proliferum group is possibly of hybrid origin involving Allium fistulosum L., the Welsh or Japanese bunching onion. Shallots were formerly considered to be a distinct species, namely Allium ascalonicum L. (Mabberley 1997).
In NW Moroccan traditional medicine, the juice from the onion bulb is applied onto eyes for ocular infections; an infusion made from the bulb is applied externally to treat haemorrhoids; and the bulb is warmed in oil and applied to skin abscesses (Merzouki et al. 2000).
In traditional Chinese medicine, the shallot is known as hsieh. A preparation of the bulb combined with honey is said to be a useful application in burns (Stuart 1911).
Onions release thiopropanal S-oxide when damaged, which is lachrymatory (Brodnitz & Pascale 1971, Freeman & Whenham 1976a). Also released is a complex mixture of sulfur-containing oils together with sulfur-free aldehydes and ammonia, all of which are more or less volatile and contribute to the odour of freshly cut onion (Boelens et al. 1971). The thiopropanal S-oxide, the volatile aldehydes, the sulfur-containing oils, etc. are produced by the onion from non-volatile precursors as a response to damage. Their principal biosynthetic precursor has been identified as S-1-propenyl-L-cysteine sulfoxide (Virtanen 1965), a sulfur-containing amino acid. This compound is almost identical with alliin, the principal flavour precursor of garlic (Allium sativum L.), which is 2-propenyl cysteine sulfoxide.
According to Vohora et al. (1973), onion applied externally acts as a rubefacient. Handling dehydrated onions has been held responsible for an irritant dermatitis (Schwartz et al. 1957). The pungent and irritant sulfur-containing oils are thought to be the causative agents (Watt & Breyer-Brandwijk 1962). Neves (1964) described a 50-year old female with eczema of the hands associated with handling onion and garlic. Patch tests with both garlic and onion showed 2+ reactions at 48 hours and 96 hours. Tests in controls were not reported.
Parasitic thrips that inhabit onion beds can attack man (Bailey 1936).
In traditional Chinese medicine, this onion is known as tsung pai or as Allii Fistulosi Bulbus. Stuart (1911) noted that buboes and abscesses are poulticed with the bulb.
This species yields the lachrymatory thiopropanal-S-oxide from its biosynthetic precursor S-1-propenyl-L-cysteine sulfoxide when damaged (Freeman & Whenham 1976b). It is widely cultivated for culinary use.
Garlic oil has been reported to induce irritation and vesication of the skin (White 1887, Burkill 1935). Nadkarni (1976) noted that the juice from garlic cloves may be used as a rubefacient liniment. Garlic juice produces superficial irritation of the skin which, although it results in rubefaction and vesication, does not penetrate the underlying tissues (Watt & Breyer-Brandwijk 1962). Vohora et al. (1973) also report that garlic applied externally acts as a rubefacient. In NW Moroccan traditional medicine, fresh cloves of garlic are used externally by friction to treat dermatitis (Merzouki et al. 2000).
Strobel et al. (1978) describe dermatitis produced by a poultice containing garlic. Neves (1964) and Martinescu (1981) have also reported contact dermatitis from the plant. In Mexico, garlic appeared to cause photosensitisation after it had been applied to a variety of dermatoses including lichen simplex and acne (Cueva & Duran 1955, Saul 1972). Lesions developed on sun-exposed sites including some to which garlic had not been deliberately applied. The clinical syndrome most commonly reported as a result of contact with garlic has been a circumscribed irritable hyperkeratotic eczema of one or both hands — often the left hand which is used to grasp the bulb firmly whilst a knife is held in the right hand (Borda & Bozolla 1961, Burks 1954, Burgess 1952). The sudden onset of eczema of this distribution has been observed in a male cook who had regularly handled garlic with impunity for over 20 years (Rook 1974). Less distinct patterns of eczema on the hands or forearms have been attributed to garlic in a 12-year old girl (Sepulveda R. 1938), and also in workers at a potato crisp factory (Inman 1965), and in a meat grinder (Edelstein 1950).
Garlic as well as onion, leek, chives, etc. produce positive patch test reactions in a high proportion of controls because of irritancy (Bleumink et al. 1972, Hjorth & Roed-Petersen 1976, Sinha et al. 1977, Pasricha & Guru 1979, Yoshikawa et al. 1979, Mitchell 1980). This complicates patch testing and renders results unreliable. Problems in patch testing were discussed by Mitchell (1980).
Garlic leaves and bulbs contain S-2-propenyl-L-cysteine sulfoxide, otherwise known as alliin (Boscher et al. 1996). Damage to the plant results in the breakdown of alliin by the co-occurring enzyme alliinase to produce allicin. The allicin then undergoes a series of disproportionation reactions and also enzymatically catalysed reductions to produce various sulfur-containing oils and allyl alcohol. These products are more or less volatile and are responsible in part for the odour of garlic (Brodnitz et al. 1971, Freeman & Whenham 1976b). Diallyl di- and tri- sulfides are the principal constituents of the resulting oil.
The allergens of garlic appear to be present in ether, ethanol, acetone, and water extracts (Burks 1954, Bleumink & Nater 1972, Hjorth & Roed-Petersen 1976, Sinha et al. 1977, van Ketel & de Haan 1978), whereas the ethanol extract but not the acetone or water extracts of onion produced a positive patch test response in a sensitised individual (van Ketel & de Haan 1978). Diallyl disulfide is considered to be the major allergen of garlic oil (Hjorth & Roed-Petersen 1976).
The way in which the plant material is prepared for patch testing also has an effect on the outcome of patch tests. This is because formation of the sulfur-containing oils and the degradation of the precursor compounds occurs only after damaging the plant material. Thus, the time allowed between cutting up or crushing the plant material and adding the chosen solvent will affect the composition of the final extract. Also of significance is the choice of solvent. Alcohol can be expected either to precipitate or to denature the enzymes involved in the breakdown of precursor compounds, thus affecting the progress of the reaction. Water, as a solvent, will also affect the progress of the reactions by diluting the reactants, but the reaction will not necessarily stop. Acetone tends not to denature enzymes but nevertheless can be expected to affect their activity by precipitating them. Of course, if the nature of the allergen is also unknown, then its solubility in the chosen solvent will be a further unknown variable.
An alternative method to solvent extraction is steam distillation. Sinha et al. (1977) suggest the use of steam distilled oils of garlic and onion because of their decreased irritancies. However, the steam distilled oils is not identical in composition with the freshly produced natural oil (Brodnitz et al. 1971, Boelens et al. 1971), and almost certainly does not contain the precursor compounds such as alliin and allicin.
When considered with the likelihood of natural variation in the levels of phytochemicals in the plant material, it is not difficult to understand the problems in producing a standardised preparation for patch testing. A filtered aqueous extract 10% in ethanol has been recommended (Yoshikawa et al. 1979, Mitchell 1980). However, controls must be tested for irritancy which is frequently observed even at this concentration.
Neves (1964) tested a 1:100 aqueous extract of garlic in a patient with eczema associated with handling garlic and onions, and observed erythema and microvesiculation as a result. Because of this he considered that alliin, since it is water soluble, was responsible for the skin reaction. Hjorth & Roed-Petersen (1976) consider the major allergen of garlic oil to be diallyl disulfide which provoked positive patch test reactions (5% in petrolatum) when applied to the skin of patients sensitised to garlic. Campolmi et al. 1982 described a 34-year old male cook who reacted to patch tests with 5% diallyl disulfide in petrolatum and with an alcoholic extract of garlic, but the reactions on controls were not reported. Bleumink & Nater (1972) prepared ethanolic and aqueous extracts of garlic and patch tested 125 persons; six reacted to the ethanolic extract, ten reacted to the aqueous extract. All of these ten subjects were women of average age 58 and all had been exposed to garlic during food preparation. A latent subclinical sensitivity was postulated.
A consideration of the phytochemistry of the two species could well serve to confirm that immunologically related substances are present in garlic and onion. However, cross reactions with onion are inconstant: Burks (1954) reported 4 from 8; Bleumink & Nater (1972) reported 0 from 1; Yoshikawa et al. (1979) reported 1 from 4; van Ketel & de Haan (1978) reported 1 from 3. The results are probably unreliable because of the frequent occurrence of irritancy.
Bleumink & Nater (1973) found sensitivity to garlic, onion, and Tulipa L., but van Ketel & de Haan (1978) could not demonstrate cross-sensitivity between tulipalin A (α-methylene-γ-butyrolactone) and garlic or onion. Cross-sensitivity between garlic and chives, onion, leek, and chicory (Cichorium intybus L., fam. Compositae) was noted by Hjorth & Roed-Petersen (1976), who also described one patient with a positive scratch test reaction to garlic indicating immediate-type hypersensitivity. Lybarger et al. (1982) investigated a male patient with asthma associated with the inhalation of garlic dust at his place of work. He also developed dyspnoea and wheezing following ingestion of garlic-containing foods. An aqueous extract of garlic gave positive scratch test reactions; the responses of 7 controls were negative. A RAST inhibition experiment with the patient's serum showed cross-allergenicity between garlic, onions, chives, and asparagus (Asparagus officinalis L., fam. Asparagaceae). Immediate hypersensitivity to garlic was also reported by Campolmi et al. (1982) who described a male cook with positive intradermal and RAST tests to garlic.
Inhibition of platelet aggregation has been shown to occur following ingestion of fresh garlic cloves (Boullin 1981). This property has been found to be associated with methylallyltrisulfide, a minor constituent of garlic oil (Ariga et al. 1981).
This species yields the lachrymatory thiopropanal S-oxide from its biosynthetic precursor S-1-propenyl-L-cysteine sulfoxide when damaged (Freeman & Whenham 1976b). An ointment prepared from chives has caused dermatitis in Japan.
A single case report of facial oedema, coughing, sneezing, and lachrymation one or two hours after handling Allium triquetrum has been reported by Black (1972). Subsequent consumption of onions (Allium cepa L.) was followed almost immediately by wheezing and a feeling of constriction in the throat. Tests for immediate-type hypersensitivity were negative, and patch tests were positive at 96 hours but not at 48 hours.
Freeman & Whenham (1976b) classified this species in a group including Allium sativum L. (see above) characterised by a high content of S-2-propenyl-L-cysteine sulfoxide, the precursor of allicin and diallyl disulfide.
Freeman & Whenham (1976b) classified this species in a group including Allium sativum L. (see above) characterised by a high content of S-2-propenyl-L-cysteine sulfoxide, the precursor of allicin and diallyl disulfide. Boscher et al. (1996) also reported the presence of S-2-propenyl-L-cysteine sulfoxide in the plant.
When applied to the skin, this species is rubefacient. Externally, the bruised leaves may be applied to abscesses and boils (Flück & Jaspersen-Schib 1976).
Boscher et al. (1996) reported the presence of S-2-propenyl-L-cysteine sulfoxide in the bulbs, inflorescential bulblets, and leaves of this species in quantities similar to those found in cultivated garlic (Allium sativum L., see above). The release of disulfides bearing allyl and methyl moieties was also demonstrated. Further, the leaves were found to contain S-1-propenyl-L-cysteine sulfoxide, the precursor of the lachrymatory principle released by onions (see Allium cepa L. above). These findings are consistent with those made earlier by Freeman & Whenham (1976b) that this species exhibits weak lachrymatory activity and that it may be classified in a group including Allium sativum L. characterised by a high content of S-2-propenyl-L-cysteine sulfoxide.
All parts of the plants, when bruised, have a strong odour of garlic (Watt & Breyer-Brandwijk 1962).