Allergies are so fascinating to me, especially considering that they are actually allergic diseases. 
I agree and have always been confused as to how they happen. What causes allergies?
Allergy
Allergies, also known as allergic diseases, are a number of conditions caused by hypersensitivity of the immune system to typically harmless substances in the environment. These diseases include hay fever, food allergies, atopic dermatitis, allergic asthma, and anaphylaxis. Symptoms may include red eyes, an itchy rash, sneezing, a runny nose, shortness of breath, or swelling. Food intolerances and food poisoning are separate conditions.
Common allergens include pollen and certain food. Metals and other substances may also cause problems. Food,
can be idiopathic (primary) or, more commonly, secondary to another disease. In the Western World, allergic or atopic diseases are the most common causes, especially those of the respiratory or integumentary systems. In the developing world, parasites are the most common cause. A parasitic infection of nearly any bodily tissue can cause eosinophilia.
Diseases that feature eosinophilia as a sign include:
- Allergic disorders
- Asthma
- Hay fever
- Drug allergies
- Allergic skin diseases
- Pemphigus
- Dermatitis