Page 1015 - Veterinary Immunology, 10th Edition

VetBooks.ir Diagnosis of Type I Hypersensitivity

Diagnosis of allergic disease is based on history, and if possible, the
identification of the offending allergens by direct skin testing or

serology and the exclusion of other similar conditions.
The term hypersensitivity is used to denote inflammation that
occurs in response to normally harmless material. For example,
animals normally do not react to antigens injected intradermally. If,
however, a hypersensitive animal is given an intradermal injection

of allergen, it will provoke inflammation (see Fig. 30.9). Vasoactive
molecules are released to produce redness (erythema) as a result of
capillary dilation, as well as circumscribed edema (a wheal) due to

increased vascular permeability. The reaction may also generate an
erythematous flare due to arteriolar dilation caused by a local axon
reflex. This wheal-and-flare response to an allergen reaches
maximal intensity within 30 minutes and then disappears within a
few hours. A late-phase reaction sometimes occurs 6 to 12 hours

later.
Intradermal skin testing using very dilute aqueous solutions of
allergens has been widely used for the diagnosis of allergies,

especially canine AD. Following injection, the site is examined for
an inflammatory response. The results obtained must be interpreted
carefully since both false-positive and false-negative responses may
occur. For example, the concentration of antigen in commercial skin
testing solutions may be too low. Dogs may be up to 10 times less

sensitive than humans to intradermal allergens such as pollens,
fungi, or danders. False-positive reactions may be due to the
presence of preservatives in allergen solutions. False-negative

responses may be due to steroid treatment. The mixture of allergens
used for intradermal skin testing commonly includes allergens from
trees, grasses, fungi, weeds, danders, feathers, house dust mites,
and insects. Intradermal skin testing is less commonly performed in
cats because they fail to develop a significant wheal, and the

reaction is therefore difficult to evaluate.
An experimental technique used to detect IgE antibodies is called
the passive cutaneous anaphylaxis (PCA) test. In this test, dilutions

of test serum are injected at different sites into the skin of a normal

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