Allergies are thought to arise out of a complex combination of genetic and environmental factors. These factors signal the immune system to produce the allergy antibody (IgE) upon exposure to a certain antigen (a pollen, food, animal dander, mold, etc), instead of classifying the antigen as harmless and ignoring it.
The IgE antibodies that are made by the immune system then find their way to “allergy cells” – such as a type of cell called a mast cell. Mast cells are found living in different body tissues (like the skin, nose, eyes, airways, stomach, etc) and can contain receptors that allow these IgE antibodies to bind to them. However, allergic reactions don’t occur just from the IgE antibodies binding to these allergy cells alone.
When the body is exposed to more allergen at a later time, the allergen particles can then find and bind to their previously made IgE antibodies that are waiting for them on allergy cells, and if enough of a load is present to bind enough IgE antibodies in a specific way (called cross-linking), the allergy cell then becomes activated. Activation of allergy cells releases natural chemicals like histamine that then go on to cause allergic reaction symptoms.