Which of the following reactions is NOT typically classified as an allergy?

When considering what constitutes an allergic reaction, it's important to recognize that allergies typically stem from an immune system response to a substance that the body identifies as foreign. Common symptoms of allergic reactions include skin rashes, itching, and respiratory issues such as difficulty breathing.

In this context, the presence of a headache does not typically fall under the classification of an allergy. Headaches can result from a variety of causes, including tension, dehydration, hormonal changes, or other medical conditions, but they are not a direct manifestation of an allergic response. Allergies primarily provoke physical reactions involving the skin or respiratory system, while a headache is more often a standalone symptom not directly linked to immune system activity in response to allergens.