Allergen Exposure

The genus Ligustrum (Privet) comprises around 20 species, subspecies and varieties. Seven of these species are native to India, and others to Europe, Asia, and northern Africa. Privet trees are naturalised in Australia and the eastern half of North America. They are now used extensively as ornamentals in urban environments.

The Privet is a semi-evergreen or deciduous shrub growing 3 to 5 m in height. The smooth bark is grey-brown. The plant is highly branched but quite irregular in shape if not sheared. The stems are stiff and erect, with grey-brown bark spotted with small brown lenticels. Leaves are dark-green and turn purplish in autumn.

It flowers from June to July in the Northern Hemisphere. The 2.5 to 7.5 cm white flowers are hermaphrodite (have both male and female organs) and are pollinated by insects. They are produced in panicles 3 to 6 cm long; each flower is creamy-white, with a tubular base and a 4-lobed corolla (4 petals) 4 to 6 mm in diameter. The flowers produce a strong, sweet fragrance that many people find unpleasant (1). The flowers are often sheared off in early summer, which releases their strong odour. The fruit is a glossy blackberry 8 mm in size, and ripens from September to October. The berries are poisonous to humans but readily eaten by birds

The tree is found in woodland, sunny edges, and dappled shade, and is grown as a hedge. The related L. ovalifolium is a very popular garden-separating hedge.

This tree has been used in basketry, to make ink and dye, and for charcoal. A yellow dye is obtained from the leaves, and a bluish-green to black one from the berries.

Allergen description

Privet pollen contains allergens of approximately 20, 18-19, 40 and 70 kDa (2-3), but only Lig v 1  (4-8) has been characterised to date.
A profilin has been isolated (9).

Potential Cross Reactivity

An extensive cross-reactivity among the different individual species of the genus could be expected, as well as to some degree among members of the Oleaceae family (10).

The main allergen of Olive tree (O. europaea), Ole e 1, has been shown to be present in other species of the Oleaceae family: White ash (Fraxinus excelsior), Privet (L. vulgare), and Lilac (Syringa vulgaris) (4). Allergens with apparent molecular weights of 49.6, 40, 36.7, 19.7, 16.7, and 14 kDa seem to be relevant (11). Studies have demonstrated that Lig v 1 plays a major role in cross-reactivity among these trees. Recombinant Fra e 1 from White ash tree has been shown to exhibit 82%, 88%, and 91% identity with Syr v 1 from Lilac tree, Ole e 1 from Olive tree, and Lig v 1 from Privet tree, respectively (12). These results are consistent with those of other authors (3,6-7,13). Further studies have confirmed the high degree of cross-reactivity among these 3 plants, and Phillyrea angustifolia, a common bush, although the allergens in the 4 species were not identical (14).

Che a 1 from Goosefoot grass has a sequence similarity of 27 to 45% with known members of the Ole e 1-like protein family (15). The significance of this potential cross-reactivity was not evaluated.

A profilin has been inferred from a study of Che a 2 and was shown to have high degrees of inhibition with profilin from pollen of other plants, e.g., Russian thistle (91%), Olive (96%), Privet (95%), Ash (95%), Lilac (93%), and Birch (94%) (9).

Profilin may be responsible for evidence of cross-reactivity between Privet tree pollen and other pollens where the responsible allergen was not identified.

In 103 atopic subjects in Michigan, USA, cross-reactivity among Olive tree, White ash, Privet and Russian olive tree pollen was demonstrated, even though the Olive tree does not grow in that area. Nineteen subjects were skin-prick positive to this allergen, confirming the cross-reactivity (16).

It has been reported that pollen from the Olive tree and Privet are cross-reactive with grass pollen from the Poaceae family (2).

Clinical Experience

IgE-mediated reactions

Privet pollen can induce asthma, allergic rhinitis and allergic conjunctivitis (17).

Privet is said not to be an important allergenic plant, as pollination is mainly by insects and pollen is not found in high levels in the air. However, as studies have demonstrated the high degree of cross-reactivity among Privet and other commonly occurring highly allergenic trees of the Oleaceae family, including Olive (Olea), Ash (Fraxinus), and Lilac (Syringa) (14), pollen-allergic individuals should be investigated for possible sensitisation to this allergen.

Authors have argued that, while Privet pollen may not account for high levels of the annual total of daily pollen concentrations measured in a city, in areas where these trees are widely used as ornamentals, the amounts accumulating may be high enough to cause allergy symptoms. Released Privet pollen grains have a short dispersal range, as a result of both the entomophilous nature of the plant and the large size of the pollen grains. In areas where Olive trees are prevalent, the last stages of the flowering period of Privet may overlap with the flowering period of Olive trees, and because the 2 pollen types share allergens, there may be a cross-reaction between Olive tree pollen and Privet pollen. It is therefore useful to test for Privet pollen sensitisation in areas where it is found concurrent with Olive tree (18). In Northern and Central Europe, where the Olive tree is not extensively cultivated, and where White ash tree and Privet are grown, positive specific IgE sensitisation tests for Olive tree may in fact indicate Privet sensitisation, as a result of cross-reactivity (17,19).

In a New Zealand study of 20 subjects, airway responsiveness (PD20 histamine) was shown to be significantly greater during the Privet flowering season. Symptom scores and bronchodilator use were higher, and peak expiratory flow rates lower, during the Privet-flowering season, but the changes were small and not statistically significant. Seventeen subjects had bronchial challenge studies performed on them; there were no isolated early responses, but 6 had late asthmatic responses. Eleven had no airway bronchial constrictor responses to challenge with either of the 2 local varieties of Privet. The authors pointed out that, although significant increases in airway responsiveness occur during the Privet flowering season, only a portion of this highly select group had a constrictor response to direct challenge (20).

A number of studies have demonstrated the presence of Privet pollen in the air of a city or region: in Salamanca in Spain (21), Huelva in southwestern Spain (with Privet pollen comprising 86% of the pollen measured) (22), and Tucson, Arizona (23). Authors have pointed out that in warmer regions of North America, many newly introduced plants, including Privet, Olive tree and Schinus, are cultivated widely, while others have become aggressive naturalised weeds (24).

Other reactions

Consumption of the fruit may result in vomiting (25).

Last reviewed: June 2022.