Anaphylaxis is a sudden, severe allergic reaction that occurs when you are exposed to a substance that your body was sensitized to during a previous exposure.
The life-threatening reaction causes a sudden release of chemicals, including histamines, from cells in the blood and the body's tissues. These chemicals dilate the blood vessels, lowering blood pressure, and they cause the blood vessels to leak fluid, leading to hives and swelling, especially around the face and throat. The chemicals also act on the lungs, causing an asthmatic reaction -- that is, causing the lung's airways to constrict, thus making breathing very difficult.
In some cases, anaphylaxis may have mild symptoms, causing only hives and itching. But it can be deadly. Blood pressure can drop severely, leading to loss of consciousness. Swelling of the throat and airways can cause difficulty breathing, speaking, and swallowing. Anaphylaxis is a medical emergency.
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