i developped an allergy last summer and it reemerged this spring however i have no idea about what's causing it but one thing is sure: it's so **** annoying

i heard there are dermic tests to identify the allergen! anyone know anything about those tests?
anyways, to add a little bit to the thread, allergy is the same as "Hypersensibilité de type 1" and it's caused by an excessive and brutal response from the immune system against a certain molecule called allergen (as carlutchi said, the allergen could be pollen, food, dust...) that normally isn't harmful to the organism. some people might be more susceptible than others to develop an allergy due to genetical factors.
When the allergen is recognized by a certain family of antibodies (IgE) binded to mastocytes cells (certain type of blood's white cells), these cells decompose and spill some substances in the circulation such as histamine causing inflammation and hence an accute immune response causing allergy.
Hypersensibilités de type 2, 3 et 4 are slower in time and are due to different mechanisms. PM for more details.