|The Biology of the Caprine Arthritis Encephalitis Virus|
|Testing for the CAE virus with AGID|
The AGID (Agar Gel Immunodiffusion) assay works under the same principle as ELISA but takes advantage of the presence of multiple types of antibodies to regions (epitopes) of the antigen. As discussed earlier the goat immune system makes antibodies to many different epitopes of the envelope protein. Currently, 6 different antibodies have been identified for the transmembrane portion of the envelope protein and 5 antibodies have been identified for the surface component of the envelope protein.
Because of these multiple epitopes, more than one type of antibody can bind to a corresponding antigen and because antibodies have two arms one antibody can sometimes bind to two antigens. If the amount of antigen to antibody is in equilibrium then large complexes of antigen/antibodies can be formed. This causes the complex to precipitate.
AGID tests are very accurate with respect to specific binding, therefore false positives are rare, but the test does not detect low levels of antibodies. This results in false negatives when the test subject is in the early stages of infection and has a low level of antibodies.
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