Final answer:
An allergic response is triggered when histamine is released by mast cells as a result of the immune system reacting to a harmless foreign substance, leading to allergic symptoms.
Step-by-step explanation:
An allergic response is triggered when a person's immune system reacts to a foreign substance, such as pollen, which is usually harmless to the body. This immune response involves the production of IgE molecules by B cells, which signals mast cells to release various chemical mediators. One of the primary chemical mediators released by mast cells during an allergic reaction is histamine. Histamine is responsible for many of the symptoms associated with allergies, such as itching, swelling, and increased mucus production. Therefore, in the context of an allergic response, histamine is absorbed, triggering various allergic symptoms.