Hypersensitivity is another name for allergy, a disorder of the immune system causing adverse reactions to substances (allergens) not harmful to most and marked by the body’s production of histamines and associated with atopy, anaphylaxis, and asthma.
What then is allergen? An allergen is any substance (antigen), most often eaten or inhaled, that is recognized by the immune system and causes an allergic reaction. Dust, pollen and pet dander are all common allergens, but it is possible to be allergic to anything, according to science news aggregation and curation site ScienceDaily.
Authors Dawn Lei and Leslie Grammer  identify other common allergens to include fungal spores, house-dust mites, and animal epithelial materials but can also include drugs, biologic products, and insect venoms. Most allergens are proteins or glycoproteins that range in molecular weight from 5000 to 100,000 Da, although polysaccharides and low-molecular weight substances may also be allergenic.
Surprisingly, even in schools, kids who don't have pets can become exposed to allergens from cats and dogs from other children and these allergens are transported on clothing and can land on textiles, according to researchers Marsa Bergel, A. K. Munirl and Sten Dreborglz  who studied the levels of secretoglobin protein, lipocalin allergen, mite allergen and American house dust mite in dust from the clothes and classrooms of children in a Swedish school to see if cat and dog owners could bring allergens to public areas in their clothes, and also investigated the levels of allergen in different areas in the four classrooms used by the children. They selected 31 children in four classes, forming three groups, namely cat owners, dog owners and children without a cat or dog at home, and detected cat and dog allergens in all 57 samples from clothes and classrooms.
So, below are 10 allergens that cause you allergy and why they should bother you:
1. Pollen (Grass, Weeds And Trees)
Pollen immunogenicity, plant abundance, proximity to living environments, and regional geography determine the specific pollens that are responsible for local allergic sensitization.
The most common cause of allergic rhinitis and asthma is grass pollen which is usually released in the afternoon. Two of the most important allergenic grasses are Rye and Timothy.
Ragweed pollen is the most important cause of allergic rhinitis and pollen asthma while pollen grains are known to trigger allergic symptoms in some parts of Australia, the United States, Canada and Europe. Weed pollen release usually occurs in the morning of autumn in the U.S.
Tree pollen is a significant cause of allergic disease, and released in most parts of Australia during a short spring season. Tree pollen allergens come from the angiosperm class while cross-reactivity among tree pollen is uncommon unlike grass pollen.
Fungi produce airborne spores and mycelial elements believed to be the main causes of allergic disease throughout the world. With a few exceptions, the clinical importance of common fungi is difficult to assess.
Alternaria alternata which triggers respiratory arrest in asthma patients and Cladosporium are common outdoor mold species. Like Alternaria, Cladosporium has a seasonal prevalence in the warmer months between spring and autumn.
3. Environmental Changes
Due to global warming, climate changes are expected to increase temperatures by 1°C to 2°C during this century, thus affecting vegetation and causing a higher allergic disease burden, with earlier onset of pollen seasons.
4. Dust Mites
Dust mites are small, eight-legged pests found in pillows, mattresses, box springs, sofas, and carpets. They thrive in warm, humid conditions. Some of them ingest human epithelial scales and obtain water from the ambient humidity, and produce feces that provide a perennial allergen source within homes.
5. Animal Aeroallergens
Animals produce forms of allergens unique to each species. The major allergen sources are dander, saliva, urine, hair, and feathers.
Cat allergen is found in cat saliva, the sebaceous glands and urine of male cats. The allergen is buoyant and can remain airborne and present in a home for up to nine months after the source is removed. Cat allergen adheres to clothes and can be found in public places like schools.
Dog allergen is present in dander, saliva, urine, and serum. It is specific to dog breeds, but all breeds produce allergenic proteins.
German and American cockroaches are the two most common species of cockroach that infest domestic homes and public buildings. The German cockroach is common in the U.S. and can be found in warm, humid environments, but cockroach infestations are also high in the inner cities.
7. Bees, Wasps And Hornets
Bees, wasps and hornets are mostly found in the warmer months, and throughout the U.S. They have nests and hives in trees, under roof eaves, or on equipment like ladders.
Medications are known for triggering undesired immunologic reactions. To cause an allergic reaction, a reactive metabolite of the drug must bind to a macromolecular carrier for antigen processing.
Sensitization in natural rubber latex is present in 75 percent of patients with spina bifida and 6.5 percent of the general population. The risk is higher in healthcare providers and patients with urologic problems that require catheterization. Clinical manifestations of IgE-mediated disease include allergic rhinitis, asthma, contact urticaria, and anaphylaxis.
10. Food Allergy
Food allergy is common and affects 15 million people in the USA, and seems to be increasing. It is divided into class 1 and class 2. Class 1 food allergy is considered “traditional” and occurs in the gastrointestinal tract. Class 2 food allergy is due to allergic sensitization to inhalant allergens.
 Lei, D. and Grammer, L., 2019. An overview of allergens. Allergy and Asthma Proceedings, 40(6), pp.362-365. https://doi.org/10.2500/aap.2019.40.4247
 Berge, M., Munir, A. and Dreborg, S., 1998. Concentrations of cat (Fel d 1), dog (Can f 1) and mite (Der f 1 and Der p 1) allergens in the clothing and school environment of Swedish schoolchildren with and without pets at home. Pediatric Allergy and Immunology, 9(1), pp.25-30. https://doi.org/10.1111/j.1399-3038.1998.tb00296.x