What are the signs of an allergic reaction in pets?

The signs of an allergic reaction in pets prominently include itching, swelling, and redness of the skin. These symptoms arise as the body's immune response reacts to allergens, such as certain foods, medications, insects, or environmental factors. The itching, or pruritus, is often accompanied by a noticeable inflammatory response, which can manifest as swelling and redness in the affected areas.

Excessive grooming can sometimes be a result of the itching caused by an allergic reaction, but it is not an independent sign of an allergy itself. Loss of appetite and lethargy can be symptoms of various health issues but are not specific to allergic reactions. They might occur as a result of the pet feeling unwell due to the allergic response but are not direct indicators of an allergy. Therefore, the combination of itching, swelling, and redness specifically points to an allergic response in pets.