Final answer:
The substance that triggers an allergic reaction is called an allergen, which causes the immune system to produce an inflammatory response, resulting in allergy symptoms.
Step-by-step explanation:
The foreign substance responsible for causing an allergic reaction is called a(n) allergen. An allergy occurs when the immune system mistakenly identifies a normally harmless substance as a threat and launches an immune response against it. During this reaction, immune cells such as B cells produce IgE antibodies that recognize and bind to the foreign protein. These antibodies are then attached to mast cells, which are primed to react upon subsequent encounters with the allergen by releasing inflammatory substances like histamines, leading to various allergy symptoms.