Page 893 - Veterinary Immunology, 10th Edition

molecules such as ICAM-1, CD4, and CD28.
VetBooks.ir Evasion of Natural Killer Cells

NK cells kill virus-infected cells at an early stage of infection before
T and B cells are fully activated. Cytotoxic T cells kill targets that

express foreign antigens on MHC class I molecules. NK cells kill
targets that fail to express these MHC class I molecules. For a virus
to survive, it must induce the selective downregulation of some

MHC class I molecules, enabling the virus-infected cells to evade
destruction by T cells while at the same time preventing NK cell
activation. Some viruses may decrease the expression of the stress-
related protein MIC-B and so inhibit NK cell mediated cytotoxicity.

Alterations in the B Cell System

One of the simplest mechanisms of viral immune evasion involves
antigenic variation of RNA viruses. Rapidly occurring point
mutations accompanied by poor editing functions of RNA

polymerases permit the generation of closely related but distinct
viruses. The most significant examples of this occur among the
influenza A viruses and the lentiviruses.
Influenza A viruses express envelope proteins called

hemagglutinins and neuraminidases. There are at least 18 different
hemagglutinins and nine neuraminidases found among the type A
influenza viruses; they are identified according to a standard
nomenclature system. The hemagglutinin of the swine influenza

virus is called H1, and its neuraminidase is called N1. The two
subtypes of the equine influenza viruses are typified by
A/equine/Prague/56, which has H7 and N7, and A/equine/Hong
Kong/92, which has H3 and N8. Highly pathogenic bird flu virus

strains include H7N9 and H5N1. H3N8 and H3N2 strains are
circulating in dogs in the United States (Table 27.1).

TABLE 27.1
Examples of Influenza A Virus Strains and Their Antigenic

Structure

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