Human Physiology, 14th edition (2016)

524 Chapter 15

Unlike IgG antibodies that circulate in the blood plasma,
IgE antibodies are concentrated in mucosal membranes. There,
the constant fragment (F c ) portions of the antibodies bind to
receptor proteins on the surface of mast cells and basophils.
When a person is again exposed to the allergen, the allergen
binds to these IgE antibodies. The allergen thereby cross-links
the IgE bound to its receptors on the mast cells and basophils,
which stimulates these cells to release histamine and other cyto-
kines (including prostaglandins and leukotrienes; chapter 11,
section 11.7) that produce the immediate hypersensitivity reac-
tions ( fig. 15.23 ).
Histamine stimulates smooth muscle contraction in the
respiratory airways (producing bronchoconstriction), but
causes smooth muscle relaxation in blood vessels (producing
vasodilation). Histamine also increases capillary permeability,
thereby promoting the exit of plasma proteins and fluid and
producing a localized edema. In addition, histamine influences
the specific immune response, promoting the release of inflam-
matory cytokines.
The symptoms of hay fever (itching, sneezing, tear-
ing, runny nose) are produced largely by histamine and can
be treated effectively by antihistamine drugs that block the
H 1 -histamine receptor. In asthma, the difficulty in breathing
is caused by inflammation and smooth muscle constriction
in the bronchioles as a result of chemicals released by mast
cells and eosinophils. In particular, the bronchoconstriction in
asthma is produced by leukotrienes, which are mainly secreted
by activated eosinophils. Asthma is treated with epinephrine
and more specific b 2 -adrenergic stimulating drugs (chapter 9)
that cause bronchodilation, and with corticosteroids, which
inhibit inflammation and leukotriene synthesis. Asthma and its
treatment are discussed more fully in chapter 16, section 16.3.
Regarding food allergies (to milk, eggs, peanuts, soy, wheat,

and others), no specific therapy is currently available. People

with a food allergy must thus be very diligent about avoiding

the particular food.

B cell Plasma cell IgE antibodies

Allergen

Mast cell

Allergen

IgE receptor

Histamine

and other

chemicals

Allergy

Granule
Figure 15.23 The mechanism
of immediate hypersensitivity. Allergy
(immediate hypersensitivity) is produced when
antibodies of the IgE subclass attach to tissue
mast cells. The combination of these antibodies
with allergens (antigens that provoke an allergic
reaction) causes the mast cell to secrete
histamine and other chemicals that produce the
symptoms of allergy.

Clinical Investigation CLUES

Timmy’s mother gave him an antihistamine for his

seasonal cough.

• What is the likely reason for Timmy’s seasonal
  cough?


• What processes produced Timmy’s cough, and how
  would an antihistamine help?

Immediate hypersensitivity to a particular antigen is com-

monly tested by injecting various antigens under the skin

( fig. 15.24 ). Within a short time a flare-and-wheal reaction is

produced if the person is allergic to that antigen. This reaction

is due to the release of histamine and other chemical media-

tors: the flare (spreading flush) is due to vasodilation, and the

wheal (elevated area) results from local edema.

Allergens that provoke immediate hypersensitivity include

various foods, bee stings, and pollen grains. The most common

allergy of this type is seasonal hay fever, which may be provoked

by ragweed ( Ambrosia ) pollen grains ( fig. 15.25 a ). People who

have chronic allergic rhinitis and asthma because of an allergy

to dust or feathers are usually allergic to a tiny mite ( fig. 15.25 b )

that lives in dust and eats the scales of skin that are constantly

shed from the body. Actually, most of the antigens from the dust

mite are not in its body but rather in its feces—tiny particles that