Anyone who lives in a town or city will have been kept awake, at some time or another, by the sound of a burglar alarm ringing endlessly in an empty shop or house. The alarm is only meant to ring if someone breaks in, but it is triggered off by some other quite innocent event, such as a strong wind or the vibrations of a passing lorry. This is more or less what happens in the case of allergies. The mast cells, which are meant to respond to invasion by parasites, are triggered off by an innocuous substance such as eggs or cow’s milk. But why should this happen?
The answer is that the body misguidedly makes IgE antibodies that fit the antigens in these substances. A very complex and intricate set of controls normally prevent the body from making IgE in response to such harmless materials, but in the allergic individual something goes awry and the control mechanisms break down.
In the case of Jane, her body had mistakenly made IgE antibodies to an antigen in peanuts, probably a protein. The strange symptoms that she experienced on eating peanuts were all produced by mediators released from her mast cells. Mast cells in the tissues of the mouth were triggered as soon as the food came into contact with them, producing symptoms almost immediately. Her lips and tongue swelled up because tiny blood vessels inside them became leaky, allowing fluid to seep out into the surrounding tissues.
The cause of Jane’s collapse (anaphylactic shock) when she ate peanuts again was a sudden drop in blood pressure, likewise produced by the mediators. This time, it seems, more IgE was present and far more mediators released. By making the blood vessels all over her body expand, and at the same time become more leaky, the mediators left her without enough blood pressure to keep the vital organs functioning.
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