Page 1053 - Veterinary Immunology, 10th Edition

VetBooks.ir Classification of Type III

Hypersensitivity Reactions

The severity and significance of type III hypersensitivity reactions
depend, as might be expected, on the amount and site of deposition
of immune complexes. One form of reaction occurs locally when
immune complexes are generated within tissues. Another form

results when large quantities of immune complexes are produced
within the bloodstream. This can occur, for example, when an
antigen is administered intravenously to an immune recipient.
Immune complexes generated in the bloodstream are deposited in

glomeruli in the kidney, and the development of glomerular lesions
(glomerulonephritis) is characteristic of this type of
hypersensitivity. If the complexes bind to blood cells, anemia,
leukopenia, or thrombocytopenia may also result. Immune

complexes may also be deposited in blood vessel walls to cause
vasculitis or in joints to cause arthritis.
It might reasonably be pointed out that the combination of an
antigen with antibody always produces immune complexes.

However, the occurrence of clinically significant type III
hypersensitivity reactions results from the formation of excessive
amounts of these immune complexes in the wrong places. For
example, several grams of an antigen are needed to sensitize an

animal, such as a rabbit, in order to produce experimental type III
reactions. Minor immune complex–mediated lesions probably
develop relatively frequently following an antibody response,
without causing clinically significant disease or tissue damage.

1053