A person who is allergic to an antigen or allergen doesn't start manifesting allergy
symptoms or allergic reaction immediately he or she is exposed to the allergens for the first time. It takes times, months to years, before one starts showing the manifestation of the allergy symptoms. The period between the first exposure to the allergen and the manifestation of the allergy symptoms is called sensitisation period. During the sensitisation period IgE is being overproduced. Also during this period, as the IgE is produced, the IgE will be coating certain explosive cells that contain degrading chemicals. After the sensitisation period, another exposure to the allergen (such as dog dander or allergen cited above) leads to reaction of the allergen with the immune cells already coated or sensitised with IgE. This results into the cells releasing the various chemicals they contain, such as histamine. These chemicals, in turn, cause inflammation and the typical allergy symptoms. This is how the immune system becomes exaggerated and move to cause an allergic reaction when stimulated by an allergen in individuals which are allergic to the allergen.