What pollen is and how it manages to produce an allergic reaction
In order to take on board how hay fever arises, it is first necessary to have an understanding of what pollen is and how it manages to produce an allergic reaction in some people.
Pollen is composed of dry particles of protein and is produced by many different species of shrubs, grasses and weeds – and also from trees that flower throughout the growing season in the UK. Grasses and flowering trees and plants primarily use air currents – the wind – to transport the male gamete (in effect, the male sex cells which carry genetic self-replicating material, or DNA) to the female part of a neighbouring plant of the same type, with the aim of fertilization. Thousands of microscopic male gametes are encased inside each grain of pollen, which in itself is so tiny it may measure only 15 to 100 microns and appear no larger than the width of an average human hair. Indeed, one pinch of pollen powder will contain many thousands of grains. The outer wall of the grain consists of a very tough substance which protects its cargo on its journey, whereas the inner layer is composed of cellulose – the main constituent of plant cell walls and vegetable fibres. The grain is also very light and dry, enabling it to be carried quite easily on the wind.
Pollen grains are usually invisible in the air. They come in a variety of sizes and shapes that are characteristic of their species, tending to be oval, spherical or disc-shaped. The outer surface of the grain can either be smooth or contain pores, grooves, granulations, spines, furrows or have a meshed appearance. Pollen grains from most grasses, for example, have one pore and no furrows; oak tree pollen contains three furrows and a pore in the centre; and birch tree pollen has three pores and no furrows. Fir trees, pines and spruces give off pollen grains that are winged to enable them to travel further.
The transfer of pollen grains to the female reproductive part of a neighbouring plant is called pollination and enables the growth of a new plant with 50 per cent of their genetic make-up from the male part of the initial plant and 50 per cent from the recipient female part of the plant. Some plants are able to self-pollinate, but the majority pollinate with another plant of the same species – a process called cross-pollination.
The majority of plants with bright flowers, such as roses, chrysanthemums and daffodils, however, are pollinated by insects, the pollen being relatively heavy, sticky and rich in protein. This attracts insects such as bees to feed on them. In due course, the pollen grains are eliminated from the insect via faeces on to neighbouring same-species plants.
Pollen from flowering plants that attract insects can form into clumps and be visible to the eye, and some people believe this to be the cause of their symptoms. It does indeed have allergenic properties, but as it is comparatively much larger and therefore heavier than wind-borne pollen, there is far less chance of it reaching the nose. Therefore, the chief culprit in triggering hay fever is wind-borne pollen
Wind-borne pollen grains are transported by air currents. They waft upwards on the morning breeze and float back down as the ground cools at the end of the day. The reason that hay fever is often worse in the early evening is this increase in numbers of pollen grains in the air around us as they return to earth. They lodge in the nasal mucous membranes and other parts of the respiratory system where they trigger an allergic reaction.
Most species of pollen have allergenic properties, with some being notorious for triggering hay fever symptoms. For example, 60 to 95 per cent of sufferers are affected by grass pollen, whereas around 20 per cent are troubled by birch tree pollen. Approximately 15 species of plants are known to be significantly allergenic and a further five less commonly cause problems. Oak tree and plane tree pollen can be highly allergenic, as can nettle pollen and ragweed (ragweed is an uncommon plant in the UK, but prevalent in the USA, Canada and parts of Europe). To ensure that some pollen reaches its target, plants that use air currents to aid pollination are more likely to produce enormous amounts of pollen, which is another reason why it is likely to be inhaled.
Wind-pollinated plants usually have insignificant flowers that are tiny and greenish-yellow in colour. It is these flowers that produce the pollen.
The exact time that a plant releases its pollen is dependent upon the local climate and the normal ‘behaviour pattern’ of the plant. As a rule, the further north a plant is, the later in the season it will release its pollen. In contrast to pollen, mould does not have a season and can release its spores at any time of the year.