Latin name: Quercus alba
Source material: Pollen
Common names: Oak, White oak, Forked-leaf white oak, Fork-leaf oak
Oaks, making up the genus Quercus, are abundant hardy trees of deciduous forests in North America, Europe, and Asia. There are approximately 500 to 600 species worldwide, 250 in the Western Hemisphere, more than 150 in Mexico, and 70 in the United States and Canada (1-2). Oak trees are subject to a good deal of variation (3-4); for example, there are 2 principal varieties of Q. robur, which are often regarded as separate species.
However, Oaks are generally classified into 1 of 2 types, White or Red. Among species, the White oak (Quercus alba) is the most common. It is widespread in northeastern America. Related species are distributed widely over the Northern Hemisphere and are found in Java and the mountains of Mexico and South America.
The White oak is a medium-sized to large spreading deciduous tree, commonly reaching 18 to 24 m in height. Individuals may grow to more than 30 m in height and exceed 1.5 m in diameter. White oak is slow-growing and may live up to 600 years. The flowers are of 2 kinds: the male, in drooping catkins 2.5 to 8 cm long; and the female, appearing later, either singly or in pairs on short stalks, and producing as fruit an acorn 1.5 to 2.5 cm long. Acorns are generally borne in pairs.
Oaks are wind-pollinated. Flowering generally occurs in spring, when the new leaves are elongating, but varies according to latitude, weather conditions, and the genetic composition of individual trees. In the Northern Hemisphere, flowering can occur at any time from late March to May or June. Three distinct waves of flowering (early, middle, and late) have been reported. Pollen is generally shed within 3 days, but light winds can accelerate shedding, and prolonged rainy weather can delay it. Oaks shed copious amounts of pollen, more than any other plant, but it generally travels less than 200 m. Oak pollens are problematic throughout most of the US.
White oak grows in rich uplands and moist bottomlands, along streams, and on hummocks, sinks, sandy plains, and dry, gravel slopes. It occurs on all upland aspects (except for ridge tops with shallow soil) and slope positions. White oak is absent on poorly drained flats, and on very wet bottomlands.
Plants may bear seed as early as 20 years of age, but generally bear fruit between 50 and 200 years of age. White oak produces good acorn crops at erratic intervals. Unlike Red oak acorns, which take 2 years to mature on the tree, White oak acorns grow from flower to maturity in a single growing season.
White Oak is an important source of wood for furniture, veneer, paneling, and flooring. Corks are made from the thick, spongy bark of the Cork oak, which occurs in the Mediterranean region. Several species yield tannins, which are used in the leather-tanning industry, and others yield dyes from their bark. White oak is currently the major source of wood for whiskey barrels.
Acorns were traditionally an important food source for many Native American peoples. White oak acorns have been described variously as sweet and edible and as slightly bitter. The acorns were boiled to remove bitter tannins. Oils obtained from pressed acorns were used to alleviate pain in the joints.
Studies suggest that White oak pollen contains multiple proteins that are potentially allergenic (5). Apparently, not all species of Oak are equally allergenic: Q. ilex pollen, although produced in considerable quantities, was not found to cause allergies in one study (6).
The following allergens have been characterised:
- Que a 1, a 17 kDa protein, a group 1 Fagales protein (7-11).
- Que a CBP, a calcium-binding protein (12).
- Que a Profilin, a profilin (9).
Que a 1 was purified from Oak pollen extract and evaluated in 16 subjects sensitised to Oak pollen. nQue a 1 showed a 58-74% sequence identity with other pathogenesis-related class 10 allergens. All subjects were sensitised to Que a 1 and Bet v 1, and 2 to profilin (11).
Some of the pollen allergens in the various species of Oak cross-react with each other, while others are unique to their own species.
In Sapporo, Japan, many Birch pollen-allergic patients complained of typical symptoms after the Birch pollen season. This has been attributed to Birch pollen-allergic individuals being affected by Oak pollinosis due to cross-reactivity between Birch and Oak pollen (13).
Natural Birch, Alder, Horn beam, Hazel, and Oak pollen contain allergens that share IgE epitopes with recombinant Bet v 1 and recombinant Bet v 2. A combination of recombinant Bet v 1 and Bet v 2 accounted for 82% of tree pollen-specific IgE in a European study. Most of the tree pollen-specific IgE antibodies were directed against rBet v 1 (9). Similarly, patients’ IgE cross-reactivity between Birch and related tree pollen allergens has been shown to be between 75% (Oak) and 95% (Alder, Hazel and Horn beam), demonstrating the clinical importance of these trees (10). Therefore, in regions where Oak tree is a dominant or significant aeroallergen, tests for sensitisation to Birch, Alder, Beech or other Fagales members may be positive as a result of cross-reactive mechanisms (14).
In inhibitory ELISA assays, IgE binding to ginkgo pollen was inhibited more than 80% by Oak, Rye grass, Mugwort, and Ragweed; 34% by Japanese hop; and 10% by rBet v 2 at 10 µg/ml (15).
Oak pollen is a major cause of allergic rhinitis and asthma (16-19).
Oak pollen affects sensitised individuals throughout the world. In Madrid, Spain, the highest level of airborne pollen from 1979 to 1993 was from the Quercus species (17%) (19), and in Salamanca, Spain, the highest quantity of pollen was from Holm oak (20). Oak pollen has also been shown to be significant in Zurich (21), Mexico City (22), Japan (23), Korea (24), Tampa, Florida (25), the Iberian Peninsula (26), Didim, in southwestern Turkey (27), and Cape Town, South Africa (28).
A study examined the impact of different trees on asthma, and the association between daily hospitalisations for asthma and daily concentrations of different tree pollens in 10 large Canadian cities, and found that as a result of an interquartile increase in daily tree pollen concentration, percent increases in daily hospitalisation for asthma were 2.32% for the group containing Quercus and Castanea (29).
Exposure to Oak dust may also lead to the development of sore throat and bronchial hyperresponsiveness (30).
Occupational asthma and rhinitis due to Oak wood dust has been demonstrated in wood workers (31-32).
Determination of Oak-specific IgE antibodies have been documented to be a useful investigation in the case of Oak-allergic individuals (33-34).
Adverse reactions may occur to other components present in/on Oak wood. Oak processing workers have experienced allergic reactions to Penicillium citrinum (35).
A 28-year-old man developed multiple episodes of fever, cough, shortness of breath, and leukocytosis, features of hypersensitivity pneumonitis, several hours after cutting live Oak and Maple trees. Fungal cultures of wood chips from Oak and Maple trees were positive for Penicillium (3 species), Paecilomyces spp., Aspergillus niger, Aspergillus spp., and Rhizopus spp. (36).
The Oak processionary caterpillar (Thaumetopoea processionea Lepidoptera) is found living on Oak trees in several European countries. The larva develops poisonous hair (setae), filled with an urticating toxin that may on contact lead to severe dermatitis, conjunctivitis, and pulmonary affection, as described in more than 40 people, including young children, who developed symptoms after resting within 20 m of an infested Oak tree. Only a few people had touched the caterpillars; the setae causing the disease can be airborne (37-38).
Compiled by Dr Harris Steinman, email@example.com.
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