Page 923 - Veterinary Immunology, 10th Edition

(occasionally monoclonal) B cell activation involving all four IgG
VetBooks.ir classes, as well as hypergammaglobulinemia, and they develop

lesions associated with type II and type III hypersensitivity. Thus
polyclonal immunoglobulin production can lead to development of

an immune-mediated hemolytic anemia, thrombocytopenia, and
the production of antinuclear antibodies. Glomerulonephritis,
uveitis, and synovitis may result from chronic immune-complex
deposition, leading to renal failure and death.

Evasion of the Immune Response

Despite their antigenicity, parasitic protozoa survive by using
evasion mechanisms acquired over millions of years of evolution.
For example, Toxoplasma gondii can avoid neutrophil attachment

and phagocytosis. Theileria parva invades and destroys T cells. Other
protozoa such as the African trypanosomes employ multiple
suppressive mechanisms such as promoting the development of

regulatory macrophages and Treg cells, reducing complement
activation, depleting dendritic cells, or stimulating the B cell system
to exhaustion. It is unsurprising that death in bovine
trypanosomiasis is commonly due to bacterial pneumonia or sepsis
as a result of this immunosuppression.

Parasite-induced immunosuppression may assist parasite
survival. For example, Babesia bovis is immunosuppressive for
cattle. As a result, infected cattle have more ticks than noninfected

animals, and the efficiency of transmission of B. bovis is enhanced. It
must be pointed out, however, that parasite-induced
immunosuppression may kill the host as a result of secondary
infection, so it is not always beneficial to the parasite. In addition to
immunosuppression, protozoa have evolved two very effective

evasive techniques. One involves becoming less antigenic, and the
other involves the ability to alter surface antigens rapidly and
repeatedly. An example of a nonantigenic organism is the encysted

bradyzoite stage of T. gondii, which, as mentioned previously, does
not appear to stimulate a host response. Some protozoa may mask
themselves with host antigens. Examples of these include
Trypanosoma theileri in cattle and Trypanosoma lewisi in rats, both
non-pathogenic trypanosomes that survive in the bloodstream

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