Page 944 - Veterinary Immunology, 10th Edition

example, schistosomes can neutralize the alternative complement
VetBooks.ir pathway by inserting decay accelerating factor (CD55) from their

host into their outer lipid bilayer. Tapeworms can secrete sulfated
proteoglycans, which activate complement in the tissue fluid.

Parasites such as Necator americanus and H. contortus secrete
calreticulin homologs that bind C1q and block its activities.

Evasion of Adaptive Responses

Helminths become progressively less antigenic as they evolve in the
presence of a functioning immune system. Presumably, natural
selection favors the survival of parasites with reduced antigenicity.
H. contortus is much less antigenic in sheep, its natural host, than in
rabbits, which it does not normally infect. Sheep therefore respond

to fewer H. contortus antigens than do rabbits.
Helminths living within tissues may reduce their antigenicity by
adsorbing host antigens onto their surface and masking parasite

antigens. This occurs in Taenia solium infestations in swine, where
the parasites are coated with IgG. Cysticerci can also adsorb MHC
molecules to their surface.
Another mechanism of immune evasion is the use of sequential
antigenic variation. Although helminths have not evolved a system

as complex as that in trypanosomiasis, gradual antigenic variation
is recognized. The cuticular antigens of Trichinella spiralis larvae
change after each molt. Even during their growth phase, these

larvae change the expression of surface antigens. Some parasites
such as Fasciola hepatica shed their glycocalyx and hence their
surface antigens when exposed to antibodies.
Some worms interfere with antigen processing. Macrophages
from schistosome-infested animals are incompetent antigen-

presenting cells. Filarial worms secrete inhibitors that block
macrophage proteases. Taenia taeniaeformis secretes taeniastatin, a
protease inhibitor that inhibits neutrophil chemotaxis, complement

activation, T cell proliferation, and IL-2 production.
Immunosuppression is a consistent feature of parasitized
animals. This may be due to the production of immunosuppressive
molecules or to redirection of immune responses toward Treg cell
production and tolerance. F. hepatica secretes proteases that destroy

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