Which of the following is a common symptom of an allergic reaction?

Localized swelling, also known as edema, is a hallmark symptom of an allergic reaction. This occurs when the immune system responds to an allergen, causing blood vessels to become more permeable and allowing fluid to escape into the surrounding tissues. This can lead to noticeable swelling in areas such as the face, lips, or extremities, which is commonly seen in allergic reactions such as those resulting from insect bites, food allergies, or contact with allergens like plants.

The other symptoms mentioned—slurred speech, rapid heartbeat, and elevated blood pressure—are not specific indicators of an allergic reaction. Slurred speech is typically associated with neurological issues, rapid heartbeat can occur due to many conditions including anxiety or stress but is not specific to allergies, and elevated blood pressure can result from stress or pain but doesn’t directly correlate with the immune system's response to allergens. Thus, localized swelling stands out as a clear and direct symptom of an allergic response.