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A sudden drop in blood pressure is a critical indicator of a severe allergic reaction, particularly in cases of anaphylaxis. This condition can cause widespread vasodilation and increased vascular permeability, leading to significant fluid loss from the vascular space and resulting in hypotension. The drop in blood pressure can be rapid and severely compromise tissue perfusion, leading to potential shock if not treated promptly.

While an increased heart rate may also occur as a compensatory mechanism in response to hypotension, it does not specifically indicate the severity of an allergic reaction as clearly as a significant drop in blood pressure does. A decreased respiratory rate may indicate respiratory distress but is not a typical vital sign change associated with severe allergic reactions; often, an allergic reaction will cause an increased respiratory rate due to airways becoming constricted. An elevated blood temperature is generally not an immediate indicator of an allergic reaction, as temperature changes are more commonly associated with infection than with allergic responses.