Veterinary Immunology - E-Book

Ian R. Tizard

• Publisher: Saunders
• Year: 2017
• ISBN:
  9780323523486
• Edition: 10
• Topic: Medicine
• Subtopic: Veterinary Medicine

Exploring the immunologic concerns of both large and small animals, Veterinary Immunology: An Introduction, 10th Edition is the only complete resource on immunology for veterinary practitioners. This new edition has been meticulously updated to continue its trend of incorporating the latest advances and topics in the field. It features a straightforward presentation of basic immunologic principles along with thorough and timely information on the most significant immunologic diseases and responses seen in domestic animals.

• Comprehensive coverage clearly explains the general principles of immunology, and provides information on the most significant immunologic diseases and immunologic responses seen in domestic animals and marine mammals.
• A wealth of clinical examples show how principles will be experienced and addressed in the clinical setting.
• Educator and student resources on Evolve feature an image collection, enhanced animations, flashcards, content updates, and a test bank for instructors.
• Improved images clarify new content and enhance your understanding.

• NEW! Updated content covers new T cell subpopulations, newly described interleukins; new approaches to cancer immunotherapy; immunology of fish; and new advances in genomics.
• NEW! Learning objectives have been added to the beginning of each chapter.
• NEW! Chapter on commensal bacterial will address the role of commensal bacteria in veterinary immunology and provide convincing explanations for previously poorly understood phenomena.
• NEW! Information on the pathogenesis and treatment of atopic dermatitis has been added to help inform veterinarians who treat pets with dermatologic conditions.
• NEW! Revised content on cancer immunology reflects the vast expansion of information that has been uncovered in the past five years.
• NEW! Expanded information on the role of nutrition in animal immunity offers a rational basis for examining data of those who claim nutritional benefits.
• NEW! Full-color histologic images replace black and white images to more effectively convey concepts.

1

Surviving in a Microbial World

Chapter Outline

The Microbial World, 2

The Defenders, 2

Physical Barriers, 2

Innate Immunity, 3

Adaptive Immunity, 4

Antibody-Mediated Immunity, 5

Cell-Mediated Immunity, 6

Mechanisms of Adaptive Immunity, 7

Where to Go for Additional Information, 7

Learning Objectives

After reading this chapter, you should be able to:

• Recognize that our environment is occupied by dense and complex populations of microbes, especially bacteria.

• Understand that these bacteria do not usually cause disease because the immune system excludes those that can cause damage.

• Understand the importance of physical barriers to exclude invaders, innate immunity to provide rapid initial protection, and adaptive immunity to provide prolonged effective resistance to infection and disease.

• Explain the differences between the innate and adaptive immune responses.

• Explain why there are two major forms of adaptive immunity: humoral and cell mediated.

• Understand that adaptive immunity directed against bacterial invaders is mediated by antibodies produced by lymphocytes called B cells.

• Understand that adaptive immunity mediated by lymphocytes called T cells is called cell-mediated immunity.

• Explain how the adaptive immune system also generates memory cells and the importance of immunological memory.

• Define innate immunity, adaptive immunity, antibody-mediated immunity and cell-mediated immunity.

The animal body contains all the components necessary to sustain life. It is warm, moist, and rich in nutrients. As a result, animal tissues are extremely attractive to microorganisms that try to invade the body and exploit these resources for themselves. The magnitude of this microbial attack can be readily seen when an animal dies. Within a few hours, especially when warm, a body decomposes rapidly as bacteria invade its tissues. On the other hand, the tissues of living, healthy animals are highly resistant to invasion since their survival depends on preventing microbial invasion. The defense of the body is encompassed by the discipline of immunology and is the subject of this book.

Because effective resistance to infection is critical, the body dare not rely on a single defense mechanism alone. To ensure reliability, multiple defense systems must be available. Some may be effective against many different invaders. Others may destroy specific organisms. Some act at the body surface to exclude invaders. Others act deep within the body to destroy organisms that have breached the outer defenses. Some defend against bacterial invaders, some against viruses that live inside cells, and some against large invaders such as fungi or parasitic worms and insects. The protection of the body therefore depends upon a complex system of overlapping and interlinked defense networks using cells and molecules that collectively destroy or control almost all invaders. Any failure in these defenses, permitting invading organisms to overcome or evade them, will result in disease and possibly death. An effective immune system is therefore not simply a useful system to have around. It is essential to life itself.

The immune system can be thought of as a set of interactive cellular and molecular networks where the presence of foreign invaders triggers changes in cellular activities and generates an expanding set of cellular and molecular responses that eventually results in elimination of the invaders and increased resistance to infection. Most of the complexity of the immune system stems from the fact that none of its pathways are truly independent. Pathways interact and intersect. Cells talk to each other by means of hundreds of different signaling molecules. Microbial invasion results not in a single response but in multiple responses involving many different cell types producing many different molecules. Collectively, it is the responses of these cells and molecules that keep us alive in a microbial world.

The Microbial World

Historically, our concerns regarding infectious diseases have caused us to regard all microbes as potential enemies. Dangerous microbial invaders include not just bacteria and viruses, but also fungi, protozoa, arthropods, and helminths (worms). Nevertheless, the real situation is much more complex. Bacteria find animal hosts to be a rich source of nutrients and a great place to shelter. As a result, enormous numbers colonize our body surfaces, especially within the intestine, in the airways, and on our skin. Most of these bacteria—our normal microbiota—do not even try to invade the body and do not normally cause damage. They share resources with us and so are regarded as commensal organisms.

The presence of this microbiota and the diversity of molecules it generates must either be tolerated or ignored if an animal is to remain healthy. An animal cannot afford to act aggressively toward its own microbiota. Any response must be carefully regulated and must not happen unless necessary for the defense of the body. The immune system is aware of the intestinal microbiota. Numerous bacterial molecules cross the intestinal epithelium and influence the immune responses. They do not, however, automatically trigger strong defensive responses unless tissue damage occurs. The response is measured, proportional, and carefully controlled. The immune system has to watch them warily, but they rarely cause trouble. In fact, they are needed for the proper digestion of food as well as a stimulus that keeps our defenses in peak operating condition.

A small number of other, more aggressive bacteria, try to invade animal tissues and do cause damage. This is normally prevented, or at least controlled, by our immune defenses. If these organisms succeed in invading the body and overcoming the immune defenses, they may cause sufficient damage that results in disease or death. On the other hand, organisms such as the viruses are intracellular parasites that can survive for only a limited time outside the animal body. These invaders will only survive if they can avoid the host's defenses for sufficient time to replicate and transmit their progeny to a new animal host. While it is essential for an animal to control invading organisms (or at least minimize damage), viruses are under even more potent selective pressure. They must find a host or die. Viruses that cannot evade or overcome the immune defenses will not survive and will be eliminated. Fungi, like bacteria, are opportunistic invaders that can take advantage of local circumstance to invade the host. They commonly exploit situations where the host's immune system is defective or suppressed in some way. Parasitic worms and protozoan parasites, like viruses, must be able to survive within a host or be eliminated. They have evolved numerous and complex strategies to evade immune destruction.

An organism that can cause sufficient damage to result in disease is said to be a pathogen. Remember, however, that only a small proportion of the world's microorganisms are associated with animals, and very few of these can overcome the body's defenses and become pathogens. Pathogenic microorganisms vary greatly in their ability to invade the body and cause damage. This ability is termed virulence. Thus a highly virulent organism has a greater ability to cause damage than an organism with low virulence. If a bacterium can cause significant damage almost every time it invades a healthy individual, even in low numbers, then it is considered a primary pathogen. Examples of primary pathogens include canine distemper virus; feline panleukopenia virus; and Brucella abortus, the cause of contagious abortion in cattle. Other pathogens may be of such low virulence that they will only cause disease if administered in very high doses or if the immune defenses of the body are impaired first. These are opportunistic pathogens. Examples of opportunistic pathogens include bacteria such as Mannheimia hemolytica and fungi such as Pneumocystis jirovecii. These organisms rarely cause disease in healthy animals.

For many years, it was believed that the role of the immune system was simply to ensure the complete exclusion of all invading microbes by distinguishing between self and not-self and eliminating foreign antigens. We now know, however, that this is insufficient to ensure health. The immune system must also determine the threat level posed by the microbes it encounters and adjust its response accordingly. It must maintain tolerance to the normal microbiota or food antigens while, at the same time, be highly responsive to invading pathogens.

The Defenders

The defenses of the body, collectively called the immune system, consist of interacting networks of cells and molecules. For descriptive purposes, it is convenient to divide these networks into discrete pathways (Fig. 1.1). Nevertheless, the reader should be aware that these biochemical and cellular pathways are extensively interlinked. No immune response is restricted to a single biochemical mechanism or pathway. The invasion of the animal body by microbes alters the behavior of many different cell types and the production of many different molecules. Understanding immunity requires an understanding of these dynamic immunological networks. These networks possess redundancies, regulatory mechanisms, and multiple simultaneous responses working together to ensure microbial destruction. In addition, the immune responses are adaptable and adjust their mechanisms depending upon the nature and severity of the threat. This of course maximizes their efficiency and minimizes the chances of any individual microbe successfully evading those defenses.

FIG. 1.1 The basic arrangement of the innate and ad...