Page 252 - Veterinary Immunology, 10th Edition

pores on the surface of Gram-negative organisms. Heat-shock
VetBooks.ir proteins are generated in large amounts in stressed bacteria. The

exotoxins are toxic proteins secreted by bacteria or released into the
surrounding environment when they die. Exotoxins are highly

immunogenic proteins and stimulate the production of antibodies
called antitoxins. Many exotoxins, when treated with a mild
protein-denaturing agent such as formaldehyde, lose their toxicity
but retain their antigenicity. Toxins modified in this way are called

toxoids. Toxoids may be used as vaccines to prevent disease caused
by toxigenic bacteria such as Clostridium tetani. Bacterial nucleic
acids rich in unmethylated CpG dinucleotides serve both as
effective antigens for the adaptive immune system and as potent

stimulators of innate immunity acting through TLRs. The presence
of the enormous and diverse intestinal microbiota provides a source
of many different microbial antigens. These too can stimulate
adaptive immune responses.

Viral Antigens

Viruses are very small structures that grow only inside living cells.
They are thus “obligate,” intracellular parasites. Viruses usually
have a relatively simple structure consisting of a nucleic acid core

covered by a protein layer (Fig. 9.2). This protein layer is termed the
capsid, and consists of multiple subunits called capsomeres. Capsid
proteins are good antigens, well capable of stimulating antibody

responses. Some viruses may also be surrounded by an envelope
containing lipoproteins and glycoproteins. A complete viral particle
is called a virion. When a virus infects an animal, its proteins are
processed, recognized, and trigger adaptive immune responses.
Viruses, however, are not always found free in the circulation but

live within cells, where they are protected from the unwelcome
attentions of antibodies. Indeed, viral nucleic acids may be
integrated into a cell's genome. In this situation, the integrated viral

genes code for new proteins, some of which are expressed on the
surface of infected cells. These proteins, although they are
synthesized inside an animal's own cells, can still bind to antigen
receptors and provoke adaptive immunity. These newly
synthesized foreign proteins are called endogenous antigens to

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