A. Airborne allergens
1.Pollen
Pollen is the male reproductive cell of seed plants. Pollen is produced and released in the pollen sac. Some of them are spread by wind and some rely on insects, the former is called wind-borne flowers and the latter is called insect-borne flowers. Wind-borne flowers are characterized by their high yield, small size, light weight and, in some cases, air sacs, so they can be dispersed over great distances. Most of the pollen used as airborne allergens belongs to wind-borne flowers.
Different plants have different flowering periods, which creates a seasonal characteristic of pollen transmission. Botany divides the flowering time into three phases: the beginning of flowering, the peak of flowering, and the end of flowering. At the beginning of flowering, the first pollen starts to be dispersed; at the end of flowering, most of the flowers have withered and the number of pollen decreases sharply.
The dispersal of pollen in the air is also geographically specific. Although some pollen can be dispersed far away, generally pollen is dispersed mainly around the surrounding area of the flowering plant. The geographical distribution of plants due to different geographic conditions and anthropogenic reasons also contributes to the regional characteristics of pollen dispersal, which is of epidemiological importance in hay fever.
Pollen dispersal is also influenced by meteorological conditions. Generally speaking, high temperature and dry environmental conditions are most suitable for pollen dispersal. Conversely, lower temperatures and higher humidity are not conducive to pollen dispersal.
Pollen is the main source of airborne allergens, causing 10-20% of allergic diseases. It can be divided into three main categories: grass pollen, weed pollen and tree pollen. The allergenic components in pollen are mainly proteins. Different pollens have different protein components and their antigenicity is not consistent. There may be cross-reactivity between different pollens.
(1) Grass pollen: There are many types of grass pollen, and their allergens are also diverse. At present, based on the physicochemical and immunobiological properties of grass pollen allergens, they are divided into ten categories of allergens. These include expansins, extensins, ribonucleases, protease inhibitors-like proteins, calcium-binding proteins and profilins. actin (profilins), etc.
(2) Weed pollen: The main weed pollen allergens that are closely related to allergic diseases are pollen from the Asteraceae family, such as Artemisia, ragweed, sunflower and chamomile. In North America and Europe, the most studied allergen is ragweed allergen, whose main protein is pectate lyase, which is related to the cleavage of pectin. For pectin degradation, the involvement of many other enzymes is required, such as polygalacturonase, polymethylgalacturonase, and epimeric polygalacturonase lyase. These enzymes have also been shown to be present as allergenic components of tree pollen.
(3) Tree pollen: Depending on its source, tree pollen can be divided into angiosperm (flowering tree) pollen and gymnosperm (pine and cypress) pollen. A variety of allergens have been extracted from tree pollen. The most important allergens in angiosperm pollen are ribonucleases, whereas in gymnosperm pollen most of them are enzymes related to pectin degradation. These differences can be seen in the differences in the structure of the two types of pollen.
2. Fungi
Fungi (fungi) are a class of lower plants without the distinction of roots, stems and leaves, and belong to the phylum Fungi. Does not contain chlorophyll, so can not use inorganic material for photosynthesis, and can only camp parasitic or saprophytic life. Most fungi have mycelium and can reproduce sexually or asexually. The fungus is easy to grow in a warm and humid environment, so its distribution also has regional differences, more in coastal areas or inland humid areas, but other areas also have distribution. Different seasons, fungal distribution is also different, generally more in summer and autumn, less in winter after snowfall. However, fungi can easily move indoors, thus becoming a perennial allergen. Allergenic fungi are mainly non-pathogenic fungi that are parasitic on decaying matter, but some fungi can both infect and allergenic to humans.
Fungi generally consist of spores and hyphae, both of which are antigenic, but spores are more antigenic and easily dispersed into the air, so fungal allergic reactions are mainly caused by fungal spores. The release of spores from different species of fungi is greatly influenced by external conditions. An extreme example is a fungus called Didymella exitialis, which releases spores only at night after rain. Allergenic fungi are mainly from Aspergillus, Mycosphaerella, Penicillium, Streptomyces, and Streptomyces. Among them, Aspergillus and Streptomyces play an important role in the development of asthma.
A large number of fungi are used in the manufacture of industrial enzymes, some of which may be potential allergens. Occupational fungal allergens are usually hydrolytic enzymes. And the allergen component of indoor fungi is often some enzymes related to glycolysis.
3. Dust mites
Dust mites belong to the phylum Arthropoda, class Arachnida, order True Mites, family Ixodes, genus Dust Mites. Only a few dust mites are associated with allergic reactions, mainly house dust mite, dust mite, dust mite, etc. The mites are divided into two parts: the carapace and the palate. The adult carapace is oval in shape, about 350 μm long, and has four pairs of legs. Females live about 100-150 days and males survive only 60-80 days. Water accounts for 81% of the mite’s body weight, and when the relative humidity drops to 50%, all dust mites die within 11 days, and house dust mites are even less able to tolerate dryness.
Human dander and grain dust are ideal feeders for mites, so mites exist in large numbers in bedding dust and grain dust. Mite carcasses, molts, and even excreta are antigenic. Clinical studies have proved that mites are important allergens causing allergic rhinitis and bronchial asthma.
4.Animal allergens
Allergens of animal origin are more common in the home and work environment. In the home environment, cats and dogs are common sources of allergens, while in some specific work environments rats, guinea pigs, horses and rabbits can also become important sources of allergens. The hair, epithelium, dander, urine, and saliva of these animals can be highly allergenic. Patients who are allergic to cats and dogs often show cross-sensitivity to different animal allergens. It is now recognized that albumin in serum can act as a relevant cross-reactive antigen.
II. Oral allergens
1. Food
Theoretically, any food has the potential to cause an allergic reaction. Common foods prone to allergic reactions are peanuts, nuts, seafood, mushrooms, milk, fruits, etc. The onset of food allergic reactions is complex, from type I to type IV allergic reactions are involved. The symptoms caused by food allergic reactions are also diverse, including gastrointestinal symptoms such as abdominal pain and diarrhea, skin symptoms such as urticaria, angioedema, eczema, and whistling symptoms such as asthma and runny nose. The most important allergens in food include small clear proteins, pro-myosin, actin, and lactic proteins. Some patients with pollen allergy can also have allergic reactions to fruits and vegetables at the same time. In particular, birch and grass pollens. In addition, common pollens that cross-react with food include beech pollen type 1 and type 2 allergens, whose components are ribonuclease and actin, respectively.
2. Medications
Most of the drugs associated with allergic diseases have a small molecular weight and are generally present in the form of antigens. The representative drugs associated with allergic reactions in the rapid-onset phase are antibiotics and anesthetics, which can induce allergic reactions in small doses. Severe reactions such as anaphylaxis are usually associated with injectable medications.
III. Injectable allergens
Exposure to allergens through the injectable route is also common. Most of them are caused by insect stings, but can also be caused by injectable medications. The main insects that can cause allergic reactions are bees, hornets, ants, and mosquitoes, of which bee venom is one of the most important allergens in this category, but it is not common to cause anaphylaxis. The venom of the above insects is similar in composition and contains mainly vasoactive amines, peptides, and several inflammatory enzymes such as phospholipase, hyaluronidase, and acid phosphatase.