The Immune System 525Figure 15.24 A skin test for allergy. If an allergen ( a ) is injected into the skin of a sensitive individual, a typical flare-and-
wheal response ( b ) occurs within several minutes.
(a) (b)
Figure 15.25 Common allergens. ( a ) A scanning electron micrograph of ragweed ( Ambrosia ), which is responsible for hay
fever. ( b ) A scanning electron micrograph of a house dust mite ( Dermatophagoides farinae ). Waste-product particles produced by the
dust mite are often responsible for chronic allergic rhinitis and asthma.
(a) (b)can enter the nasal mucosa, much like pollen grains. There can
be more than 100,000 mite feces per gram of house dust!
Delayed Hypersensitivity
In delayed hypersensitivity, as the name implies, symptoms
take a longer time (hours to days) to develop than in immediate
hypersensitivity. This may be because immediate hypersensitiv-
ity is mediated by antibodies, whereas delayed hypersensitivity
is a cell-mediated T lymphocyte response. Because the symp-
toms are caused by the secretion of lymphokines rather than by
the secretion of histamine, treatment with antihistamines pro-
vides little benefit. At present, corticosteroids are the only drugs
that can effectively treat delayed hypersensitivity.