What causes allergies?

Genes + environment. Noses run in families - if both parents have allergies the chance that their child will be allergic is 80%. If neither parent is allergic the child's risk is10%. Exposure to urban pollutants like diesel exhaust, cigarette smoke, & antibacterial hand cleansers upset the ecological balance between our immune system & beneficial microorganisms. With both factors present allergy is more likely.
Hypersensitivity. To what is innocuous to non-allergic subjects.
Allergens! Allergens can be anything eg pollen, dust, dander, food, drugs, etc, that causes an immune response in your body. Antihistamines & steroids attempt to mitigate that response. Check out http://www.Aaaai.Org/conditions-and-treatments/allergies.Aspx.
Antibodies. The true definition of an allergy is an ige-mediated reaction. Ige is one of 5 antibodies the human body produces. The word "allergy" itself has become corrupted over the years to signify any adverse reaction. But the true definition of an allergy is an ige-mediated (allergic antibody) reaction. See an allergist to determine if you have allergies.
Allergies. An allergic antibody called ige is responsible for the majority of allergic reactions although there are other contributing factors. It triggers an allergic cascade with the prime factor released being histamine.
IgE. Ige is a protein in the blood that is formed by the immune system. Ige specific to an allergen or an allergy producing substance can set up an allergic reaction. One must be sensitized during an initial exposure an then be re-exposed to show symptoms of allergy.
Foreign proteins. Allergies are triggered by foreign proteins which are not harmful but interact with our immune system in a way that triggers an allergic immune response. Genetic factors play a role, but it must be coupled with an environmental exposure. The allergic immune response normally targets parasites. There are likely other environmental factors (e.g. Pollution) which trigger this abnormal response.