What is an overreaction of the immune system to an unwanted substance called?

An overreaction of the immune system to an unwanted substance is referred to as an allergy. This occurs when the immune system mistakenly identifies a harmless substance—such as certain foods, pollen, or pet dander—as a threat. In response, the body produces an immune response that can lead to various symptoms, ranging from mild (like hives or nasal congestion) to severe (such as anaphylaxis).

In contrast, intolerance generally refers to a non-immune system response to certain foods, causing digestive issues instead of an immune response. Contamination involves the introduction of harmful substances, such as pathogens or toxins, into food, affecting safety rather than prompting an immune reaction. Food sensitivity often overlaps with intolerance and can include varying degrees of responses that do not fit the classical immune reaction seen in allergies.

Overall, the distinction lies in the involvement of the immune system with allergies, where a specific immune mechanism is activated inappropriately against a perceived threat.