Page 973 - Veterinary Immunology, 10th Edition

chemokines from many different cell types. It stimulates the
VetBooks.ir It is probably therefore a major contributor to the development of
production of eosinophils. It acts on sensory neurons to induce itch.

type I hypersensitivity.

IL-33 promotes mast cell adherence to blood vessel walls,
increases their IgE-mediated degranulation, and enhances their
cytokine production. Additionally, it induces the release of

leukotrienes and cytokines, and as a result, can trigger acute allergic
attacks in the absence of antigen. Similarly, it stimulates basophils
to produce their Th2 cytokines. As a result of this enhanced

cytokine release, IL-33 recruits and activates eosinophils resulting
in eosinophil infiltration (see also Chapter 28). IL-33 binds to
sensory neurons triggering intense pruritus and enhancing
scratching behavior. Levels of IL-33 are elevated in the blood of
humans suffering anaphylactic shock, in helminth infections, in

atopic human tissues, and in the lungs of those with severe asthma.

Regulation of Mast Cell Degranulation

Mast cells express two catecholamine receptors called the α- and β-
adrenoceptors. These receptors have opposing effects. Molecules
that stimulate the α-adrenoceptors (such as norepinephrine and
phenylephrine) or block the β-adrenoceptors (such as propranolol)
enhance mast cell degranulation (Table 29.2). Conversely,

molecules that stimulate the β receptors or block the α receptors
inhibit mast cell degranulation. β-stimulators include isoproterenol,
epinephrine, and salbutamol and are widely used in the treatment

of allergies. β-receptor blockers enhance mast cell degranulation
and promote allergies. Some respiratory pathogens such as
Bordetella pertussis and Haemophilus influenzae can cause β-blockade.
As a result, the airways of infected animals are more likely to
become inflamed because of mast cell degranulation. These

infections may also predispose animals to the development of
respiratory allergies.

TABLE 29.2
Effects of Stimulating α- and β-Adrenoceptors

System α Receptor Stimulation or β Blockade β Receptor Stimulation or α Blockade

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