Why will I be able to go close to cats and smell the saint from flowers without any after effect but you cannot do the same without having a cascade of reaction, why will something as innocent as cat hair make your body react in terrible ways that can be threatening to your survival? This is just a case of mistaken identity, you suffering for what your body see as evil when it isn't.
People who have environmental allergies have to go through a lot when they are exposed to them. Personally, I do not have one so I cannot tell you that I have a first hand experience of it because I do not but the people I see who have these allergies go through a lot and their signs are very visible so I can only imagine the symptoms they are experiencing. This happens because the body misidentify non-harmful substances as pathogens.
flickr: noricum
In response to the misread pathogen, the body produces an antibody which is a defense mechanism known as IgE which is shaped to bind to the allergen which it believes is a pathogen, and the IgE keeps memory of these allergens so the next time you come in contact with the allergens, the IgE antibodies decide to take action against it, your immune cells releases histamine along with a lot of other things.
Histamine is responsible for the itchy hives as a result of dilating the blood vessels in the body which causes flushing, swelling, and itching. The blood vessel dilation causes blood pressure to be decreased and heart rate increased which can lead to dizziness or fainting. In some cases, airways can even be affected when this IgE triggers the immune system to respond to the misinterpreted substances that aren't harmful.
Why do people experience allergy? Some scientists say that over cleanliness and over hygiene is the cause of allergies and this is what leads to the hygiene hypothesis. One of my friend agree to this, he always say that poor people's children from this part of the world rarely do not suffer from allergies. The hypothesis explains that we are not being exposed to enough germs that can help us build our immune system to become stronger, leaving us with fewer germs and fewer germs means more allergies.
But this hypothesis is quite complicated because being exposed to germs early in life is not good for us. Also, being exposed to diseases like mononucleosis or mumps virus can lead to health complications later in life causing allergies. So this hypothesis isn't doing justice to the answers we need, so here's another hypothesis.
The old friends hypothesis is another one. Similar to the hygiene hypothesis, this one explains that unlike in the past where we are able to reach out for a lot of things as infant, thereby allowing the body to differentiate what is good and what is bad, today this is not the case. Children are less exposed to microbes unlike in the past leading to misinterpretation of these microbes as they grow up as harmful.
Another hypothesis is Eczema and allergy. It is believed that children with eczema are likely to develop food allergies and this is because the skin with eczema in infants allows for entrance of some molecules through the skin region leading to the body identifying the molecules as foreign even when it isn't foreign. This hypothesis explain that the mode of entry for food allergies are the reason for a lot of people experiencing the allergy. If the food enters through the mouth, the body accepts it as friend but when the molecule enter through the skin, the body reads it as foe and attacks it the next time it sees it.
So why don't we prevent eczema? Well, scientists are working around this because being able to stop eczema means that a lot of food allergies can be preventable but for now, we still haven't learn about allergens and allergies completely or how to prevent them from happening to people.
For Further Reading
https://www.hopkinsmedicine.org/health/conditions-and-diseases/allergies-and-the-immune-system
https://www.nhs.uk/conditions/allergies/
https://acaai.org/allergies/allergies-101/who-gets-allergies/children/
https://www.nhs.uk/conditions/food-allergy/
https://www.fda.gov/food/buy-store-serve-safe-food/food-allergies-what-you-need-know
https://www.niaid.nih.gov/diseases-conditions/food-allergy-causes-prevention
https://www.ncbi.nlm.nih.gov/books/NBK27117/
https://www.chop.edu/conditions-diseases/ige-mediated-food-allergies
https://acaai.org/allergies/allergic-conditions/skin-allergy/eczema/
https://aafa.org/allergies/allergy-symptoms/skin-allergies/eczema/
https://eczema.org/information-and-advice/triggers-for-eczema/allergy-and-eczema/