In the case of asthma, it is the effect of histamine on the smooth muscles of the bronchi that produces the symptoms. These vital tubes, which carry air to the lungs, go into a spasmodic contraction. How the allergens reach the airways, and the types of allergens involved, will be dealt with in Chapter Three. The way in which mast cells cause other allergic reactions, such as hay-fever and perennial rhinitis, will also be described there.
As one might expect, people with these allergic disorders tend to have a higher level of IgE in their blood than others. But there are a few whose IgE levels are normal. Conversely, there are quite a large number of people who have high levels of IgE, and give positive skin-prick tests to common allergens (see box), but who display no symptoms. Perhaps these symptom-free individuals have fewer, or less accessible, mast cells than others, making them less susceptible to high IgE levels. Or perhaps the mechanisms behind allergy are more complex than they appear, and IgE is only part of the story.
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