>>1534649>>1534924>>1535870>>1538203>>1538967Allergies are caused by an immune reaction, just like when you get a bacterial or viral infection, but they involve a different kind of antibody, IgE (the usual one is IgG) and don't cause the same kind of reaction as bacteria or viruses.
IgE hangs on the outside of mast cells, and if it binds to the allergen, the mast cell busts open and releases stuff like histamine that gives you the sneezing, rash, etc.
Most people won't have an IgE-mediated reaction unless they come down with worms or something. But if you are unlucky you can be born with IgEs that are specific for normal stuff like grass or peanuts.
