What triggers an allergic reaction according to the immune system?

The immune system triggers an allergic reaction primarily through the release of histamine in response to an allergen. When a person with a sensitized immune system encounters a specific allergen—such as pollen, certain foods, or pet dander—the immune system mistakenly identifies this harmless substance as a threat. This leads to the activation of mast cells and basophils in the bloodstream, resulting in the release of histamine and other chemicals.

Histamine is a potent mediator that causes a variety of symptoms associated with allergic reactions, such as swelling, redness, itching, and mucus production. This physiological response is part of the body's defense mechanism, even though it is inappropriate in the case of allergens. By understanding the role of histamine in allergic reactions, one can appreciate how the immune system behaves inappropriately when exposed to non-threatening substances.

The other options relate to different aspects of immune function or responses that do not precisely define the mechanism of allergic reactions. Immediate hypersensitivity to food pertains to food allergies but does not encompass the entire process of how allergens trigger an immune response. A slow response to environmental changes isn't directly related to the immediate allergic reaction process, as allergic responses are typically swift. Lastly, a lack of bodily immunity refers to a compromised immune state rather