Page 1016 - Veterinary Immunology, 10th Edition

animal. After waiting 24 to 48 hours, the antigen solution is
VetBooks.ir administered intravenously. In a positive reaction, each injection

site will show an immediate inflammatory response. The injected
antibodies may remain fixed in the skin for a very long period. In

the case of the calf, this may be up to 8 weeks. Because it is
sometimes difficult to detect very mild inflammatory responses,
they can be made more visible by injecting the test animal
intravenously with Evans blue dye. The dye binds to serum

albumin and does not normally leave the bloodstream. At injection
sites where vascular permeability is increased, the dye-labeled
albumin enters the tissue fluid and forms a striking blue patch (Fig.
30.12).

FIG. 30.12 PCA reactions in a calf. Several different sera were
tested for PCA activity on the flank of a normal calf. (Courtesy Dr. P.
Eyre.)

Serological methods of measuring the level of specific IgE in
body fluids include the RAST (radioallergosorbent test), Western
blotting, and ELISA (Chapter 42). These are not subject to clinical

bias, but there has been a poor correlation between the results
obtained by serology or skin testing and clinical severity. There is
also a poor correlation between ELISA results and intradermal
testing. Serological assays are especially prone to a high level of

false-positive results (low specificity). A negative ELISA will

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