w22 Japanese Hop

Allergen Exposure

Humulus is found in temperate and subtropical regions of the northern hemisphere. There are only three species: European hop, H. lupulus, found throughout Europe; Japanese hop, H. scandens (syn. H. japonicus), found in Japan and throughout most of China; and H. yunnanensis, native to Yunnan province. (1, 2, 3, 4) Both H. lupulus and H. scandens are now found throughout other parts of the world, including the eastern United States and Canada west to Manitoba. (5, 6)

Japanese Hop is an annual or weakly perennial climber (climbing twining vine), growing 3 to 6m high. It has deeply 5- to 7-lobed large leaves with serrate edges and rough surface, with pubescent underside, on a long petiole and prickly stems. The flowers are green spikes. Variegated forms are common.

Hop is dioecious, with separate male and female plants. Male flowers are yellow-green, arranged on 15-25 cm long, narrowly spreading panicles. Female flowers are catkin-like drooping spikes 5 mm in diameter. It is entirely wind-pollinated and a large amount of pollen is produced. (1, 6) Anthesis is in later July through mid-September in the U.S. and middle Europe, and July into October in China and Korea. (1)

Japanese Hop is primarily a weed of pastures, hayfields, and other non-crop areas. It is found throughout Virginia, Tennessee, North Carolina, and West Virginia of the USA, as well as being widespread in both rural and urban areas of the Far East. It is one of the best twining climbers for screening off fences and other unsightly parts of the garden. Hop Japanese pollens are abundant in the air during the autumn season. In Korea, pollen from this tree is a particularly prominent autumn allergen. (7)

In China and Japan this plant is commonly utilised as a tonic for the genito-urinary system. The young leaves and young shoots are cooked and eaten.

Allergen description

Proteins of 10, 16, 20, 29 and 42 kDa have been isolated from H. japonicus in immunoblot analysis. In sera of H. japonicus-reactive patients, a protein of 10 kDa was the most prevalent allergen isolated; occurring in 72% of sera and therefore being classed as a major allergen. (8) An earlier study reported isolating 12 IgE-binding components ranging from 13 to 89 kDa. Three protein bands of 13, 74, and 80 kDa were isolated and bound to serum IgE of more than 50% of patients allergic to this pollen. (7)

The following allergens have been characterised:

• Hum j 1. (9, 10, 11)
• Hum j 2, a profilin. (10, 12)
• Hum j 10kD, a 10 kDa protein. (8)

Potential Cross Reactivity

An extensive cross-reactivity among the different individual species of the family could be expected. (13) Members of this family include Hemp (Cannabis sativa) and commercial Hop (Humulus lupulus)

Patients allergic to Japanese Hop pollen were noted to have an associated sensitisation to Hop (Humulus lupulus), Celery and Sunflower pollen. (14)

Ginkgo pollen, a prominent aeroallergen in Korea, was shown to have a minor degree of cross-reactivity with pollen from Japanese Hop. In patients with IgE antibodies to Ginkgo pollen, in inhibitory ELISA tests, IgE binding to Ginkgo pollen was inhibited by more than 80% by Oak, Ryegrass, Mugwort, and Ragweed; and 34% by Japanese Hop. In inhibitory immunoblot tests, IgE binding to Ginkgo pollen proteins was almost completely inhibited by Oak, Ryegrass, Mugwort and Ragweed, but only partially by Japanese Hop and rBet v 2. (15)

Cro s 2, the profilin from saffron, was shown to have the highest degree of identity and similarity to profilin from Pineapple, Hazelnut, Banana and Hazel tree (79%), and a 78% identity with Latex, Apple fruit and pollen from Japanese hop and Olive tree. (16)

Clinical Experience

IgE-mediated reactions

Japanese Hop is a very frequent cause of symptoms of asthma and allergic rhinitis in sensitised individuals in autumn, in particular in the East. Between 6.1% and 14% of Korean patients with asthma, rhinitis and conjunctivitis attending an allergy clinic have been shown to be sensitised to this allergen, and a similarly high prevalence occurs in China. (7, 17, 18, 19, 20, 21) Recent studies in Korea suggest that sensitisation to this pollen is increasing. (14)

Although exposure to pollen from this plant appears to be less prevalent in Europe and the USA, aerobiological studies have detected this pollen in the city of Salamanca, Spain, as well as on the western United States Gulf Coast. (22, 23)

In Korean apple farmers, Japanese Hop was demonstrated to be a prevalent allergen, although the most common sensitising allergen was European red spider mite (23.2%), followed by Tyrophagus putrescentiae (21.2%), two-spotted spider mite (16.6%), Dermatophagoides farinae (16.3%), D pteronyssinus (14.4%), and cockroach (13.1%). Twelve percent of the study group were found to be sensitised to Japanese Hop pollen. (24)

Other reactions

Humulus japonicus vines are covered with hooked hairs which make working with them painful. Dermatitis and blistering may occur.

Hop (Humulus lupulus) f324 are the dried flower heads or the extract of the fruit of another Hop plant and are used in beer manufacture. Respiratory symptoms are common among Hop pickers, who may develop a contact urticaria or dermatitis from the leaves, as well as conjunctivitis and tenosynovitis, which is felt to be irritant rather than allergic, and due to the myrcene oxidation products, humulone and lupulone. (1, 25, 26, 27)

Last reviewed: June 2022.