Animal Immune System PDF

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 b...

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 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. Physical Barriers Because the successful exclusion of microbial invaders is essential for survival, it is not surprising that the animal body employs multiple, overlapping layers of defense (Fig. 1.2). For example, a microbe that has succeeded in breaking through the first layer of defenses is then confronted with the need to overcome a second, higher barrier, and so forth. The first and most obvious of these defenses are the physical barriers to invasion. Intact skin provides an effective barrier to microbial invasion. If skin is damaged, microbes may invade; but wound healing ensures that this is repaired promptly. On other body surfaces, such as in the respiratory and gastrointestinal tracts, simple physical defenses include the "self-cleaning" processes: coughing, sneezing, and mucus flow in the respiratory tract; vomiting and diarrhea in the gastrointestinal tract; and urine flow in the urinary system. The presence of huge populations of commensal bacteria on the skin, respiratory tract, and in the intestine also excludes many potential invaders. Well-adapted commensal organisms adapted to living on body surfaces can easily outcompete poorly adapted pathogenic organisms. The microbiota thus plays an essential role in resistance to invasion. Innate Immunity Physical barriers, though essential in excluding invaders, cannot be totally effective by themselves. Given time and persistence an invading microorganism will eventually overcome mere physical obstacles. Nevertheless, most microbial attempts at invasion are rapidly blocked before they can result in disease. All animals and plants, even the least evolved, need to detect and eliminate microbial invaders as fast and as effectively as possible. This immediate response is the task of the innate immune system. Many different innate defense mechanisms have evolved over time and the mammalian innate immune system is therefore a diverse collection of subsystems that work through many different mechanisms. Collectively, they all respond rapidly to block microbial invasion and minimize tissue damage (Fig. 1.3). Innate immune responses are activated immediately when a pathogen penetrates the epithelial barriers. These responses are generic; that is, they detect microbes such as bacteria and viruses because they differ structurally and chemically from normal animal tissues. Once the invaders are recognized, multiple innate responses are available to destroy them. For example, animals make many different antimicrobial proteins that either kill invaders directly or promote their destruction by defensive cells. Some of these molecules are present in normal tissues all the time, while others are produced in response to the presence of bacteria, viruses or cell and tissue damage. Other innate subsystems rely on rapid cellular responses to invasion. Thus the body employs sentinel cells that can detect invading bacteria and viruses. Sentinel cells recruit other cells, called leukocytes, that converge on the invaders and destroy them in the process we call inflammation. Inflammation is central to the innate defenses of the animal body. Some of the cells involved in inflammation may also help repair damaged tissues once the invading microbes have been destroyed. It is the presence of a combination of microbial-induced tissue damage as well as inflammation that results in the set of animal behaviors that we call sickness. The innate immune system is a mixture of "hard-wired" subsystems that lack any form of memory, and, as a result, each episode of infection is treated identically. The intensity and duration of innate responses such as inflammation therefore remain unchanged no matter how often a specific invader is encountered. These responses also come at a price: the pain of inflammation or the development of sickness largely result from the activation of innate immune pathways. On the other hand, the multiple subsystems of the innate immune system are "on call" and ready to respond immediately when invaders are detected. Adaptive Immunity Inflammation and the other innate defenses are critical to the defense of the body. Animals that fail to mount innate responses will die from overwhelming infections. Nevertheless, these responses cannot offer the ultimate solution to the defense of the body. What is really needed is a defense system that can recognize and destroy specific invaders, and then learn from the process so that if they invade a second time, they will be destroyed even more effectively. In this system, the more often an individual encounters an invading bacterium or virus, the more effective will be its defenses against that organism. This type of "smart" response is the function of the adaptive immune system, so called since it adapts itself to ongoing threats to the animal. Although it develops slowly, when an animal eventually develops adaptive immunity, the chances of successful invasion by that organism decline precipitously, and the animal is said to be immune. The adaptive immune system provides the ultimate defense of the body. Its essential nature is readily seen when its loss leads inevitably to uncontrolled infections and death. A key difference between the innate and adaptive immune systems lies in their use of cell surface receptors to recognize foreign invaders (Table 1.1). The cells of the innate system use a limited number of preformed receptors that bind to molecules expressed by many different microbes, and the response is therefore generic in nature. In contrast, the cells of the adaptive immune system generate enormous numbers of new, structurally unique receptors that bind specifically to those foreign molecules that induce them. Because the binding repertoire of these receptors is generated randomly, they are assured of recognizing at least some of the molecules found on almost any invading microorganism. The adaptive immune system not only recognizes invading microbes, but it also destroys them and retains the memory of the encounter. If the animal encounters the same organism a second time, the adaptive immune system responds more rapidly and much more effectively. Such a sophisticated system must, out of necessity, be complex. Another reason for this complexity is the great diversity of potential invaders, including: bacteria, viruses, fungi, protozoa, and helminths (worms). These invaders may be classified into two broad categories. One category consists of the organisms that normally reside outside cells---extracellular invaders. This includes most bacteria and fungi, as well as many protozoa and invading helminths. The second category consists of organisms that originate or live within the body\'s own cells ---the intracellular invaders. These include viruses and intracellular bacteria or protozoa. Each category requires a different defensive strategy. The adaptive immune system thus consists of two major branches (Fig. 1.5). One branch is directed against the extracellular invaders. The other is directed against intracellular invaders. Both branches depend upon the use of specialized white blood cells called lymphocytes. There are two major lymphocyte populations, B cells and T cells. Immunity to extracellular invaders is mainly the function of B cells. They produce proteins called antibodies that promote the microbial destruction. This B-cell-mediated immune response is sometimes called the "humoral immune response" since antibodies are found in body fluids (or "humors"). Antibody-Mediated Immunity Soon after it was discovered that animals could be made immune to infectious agents by vaccination, (Chapter 24) it was recognized that the substances that provided this immunity could be found in blood serum. For example, if blood is taken from an immune horse that has been previously vaccinated against tetanus (or has recovered from tetanus), its serum separated and then injected into a normal horse, the recipient animal will become temporarily resistant to tetanus. The protective molecules found in the serum of immune animals are proteins called antibodies. Antibodies against tetanus toxin are not found in normal horses but are produced following exposure to tetanus toxin as a result of infection or vaccination. Tetanus toxin is an example of a foreign substance that stimulates an adaptive immune response. The general term for such a substance is antigen. When an antigen enters an animal, the animal\'s B cells are stimulated to produce antibodies that bind to that antigen and ensure its destruction. Antibodies are highly specific and bind only to the antigen that stimulates their production. For example, the antibodies produced in response to tetanus toxin bind only to tetanus toxin. When the antibodies bind, they "neutralize" the toxin so that it is no longer toxic. In this way antibodies protect animals against lethal tetanus. The time course of the antibody response to tetanus toxin can be examined by taking blood from a horse at intervals after injection of a low dose of the toxin. The blood is allowed to clot and the clear serum removed. The amount of antibody in the serum may be estimated by measuring its ability to neutralize a standard amount of toxin. Following a single injection of toxin into an unexposed horse, no antibody is detectable for several days (Fig. 1.7). This lag period lasts for about 1 week as responding B cell populations grow and begin to produce antibodies. When antibodies eventually appear their levels climb to reach a peak by 10 to 20 days before declining and disappearing within a few weeks. The amount of antibody formed, and therefore the amount of protection conferred, during this first or primary response is relatively small since there are few antibodyproducing B cells. However, memory B cells are produced in large numbers. If sometime later a second dose of toxin is injected into the same horse, it is recognized by this much larger population of memory B cells. As a result, the lag period lasts for no more than 2 or 3 days. The amount of antibody in serum then rises rapidly to a high level before declining slowly. Antibodies may be detected for many months or years after this injection. A third dose of the antigen given to the same animal results in an immune response characterized by an even shorter lag period and a still higher and more prolonged antibody response. As will be described later in this book, the antibodies produced after repeated injections are better able to bind and neutralize the toxin than those produced early in the immune response. This progressive improvement of adaptive immune responses to infectious agents by repeated injections of antigen effectively generates memory cells and forms the basis of vaccination. The response of an animal to a second dose of antigen is very different from the first in that it occurs much more quickly, antibodies reach much higher levels, and it lasts for much longer. This secondary B cell response is specific in that it can be provoked only by a second dose of the same antigen. A secondary response may be provoked many months or years after the first injection of antigen, although its size tends to decline as time passes. A secondary response can also be induced even though the response of the animal to the first injection of antigen was so weak as to be undetectable. These features of the secondary response indicate that memory B cells possess the ability to "remember" previous exposure to an antigen. For this reason, the secondary immune response is sometimes called an anamnestic response (anamnesko is Greek for "remembering"). Cell-Mediated Immunity If a piece of living tissue such as a kidney or a piece of skin is surgically removed from one animal and grafted onto another of the same species, it only survives for a few days before being rejected and destroyed by the recipient. This process of graft rejection is significant because it demonstrates a mechanism whereby foreign cells, differing only slightly from an animal\'s own normal cells, are rapidly recognized and destroyed. Even cells with minor structural abnormalities may be recognized as foreign by the immune system and destroyed, though they are otherwise apparently healthy. These abnormal cells include aged cells, virus-infected cells, and some cancer cells. The immune response to foreign cells as shown by graft rejection is mediated by cytotoxic T cells that identify and destroy the "abnormal" cells. If a piece of skin is transplanted from one dog to a second, unrelated dog, it will survive for about 10 days. The grafted skin will initially appear to be healthy, and blood vessels will develop between the graft and its host. By 1 week, however, these new blood vessels will begin to degenerate, the blood supply to the graft will be cut off, and the graft will eventually die and be shed (Fig. 1.8). If the experiment is repeated and a second graft is taken from the original donor and placed on the same recipient, then the second graft will survive for no more than a day or two before being rejected. Thus the rejection of a first graft is relatively weak and slow and analogous to the primary antibody response, whereas a second graft stimulates very rapid and powerful rejection similar in many ways to the secondary antibody response. Graft rejection, like antibody formation, is a specific adaptive immune response in that a rapid secondary reaction occurs only if the second graft is from the same donor as the first. Like antibody formation, the graft rejection process also involves the generation of long-lived memory cells, since a second graft may be rapidly rejected many months or years after loss of the first. However, graft rejection differs from antibody-mediated immunity in that it cannot be transferred from a sensitized to a normal animal by serum. The ability to mount a secondary reaction to a graft can only be transferred between animals by living T cells. These T cells are found in the spleen, lymph nodes, or blood, and they are responsible for organ graft rejection. It is a good example of a cell-mediated immune response. Mechanisms of Adaptive Immunity In some ways the adaptive immune system may be compared to systems in a totalitarian state in which foreigners are expelled, citizens who behave themselves are tolerated, but those who "deviate" are eliminated. While this analogy must not be carried too far, clearly such regimes possess a number of characteristic features. These include border defenses and a police force that keeps the population under surveillance and promptly eliminates dissidents. In the case of the adaptive immune system, the antibody-mediated responses would be responsible for keeping the foreigners out, whereas the cellmediated responses would be responsible for stopping internal dissent. Organizations of this type also tend to develop a pass system, so that invading foreigners or dissidents not possessing certain identifying features are rapidly detected and dealt with. Similarly, when foreign antigens enter the body, they first must be trapped and processed so that they can be recognized as being foreign. If so recognized, then this information must be conveyed either to the antibody-forming B cells or to the T cells of the cellmediated immune system. These cells must then respond by producing specific antibodies and/or cytotoxic T cells that can eliminate the antigen. The adaptive immune system must also generate long-lived B or T memory cells that can remember this event so that the next time an animal is exposed to the same antigen, these cells will respond faster and with greater efficiency. In our totalitarian state analogy, the police force would be trained to recognize selected foreigners or dissidents, keep a file on them, and respond more promptly when they reappear. It must be emphasized, however, that just as human societies and responses are very complex and involve the interactions of thousands of individuals, so too is the immune system. While, for reasons of simplicity and teaching, we discuss discrete cells, processes and pathways, the immune systems should be thought of as an interactive network. Thousands of different cells interact in many ways and are subject to multiple influences. The cells involved interact with each other, sometimes in a very complex manner. Likewise, the invading microbe, its virulence, its ability to evade defenses, and its interactions with other microbes, all lead to variations in a host\'s immune response. For introductory purposes, we can consider the process of adaptive immunity to proceed by a series of steps (Fig. 1.9). Thus it is triggered by cells that can recognize, trap, and process antigen. The most important of these cells are dendritic cells and macrophages. These cells then present the antigen to the T and B cells of the immune system. The T and B cells can recognize and respond to this processed antigen since they possess specific antigen receptors on their surface. The B cells, once activated, will produce specific antibodies, while the T cells will participate in the cell-mediated immune responses. Longlived B and T memory cells are generated at the same time. These cells retain the memory of these events and will react very rapidly to each specific antigen if it is encountered again. They are thus responsible for the enhanced immunity that develops in secondary immune responses. Finally, helper and regulatory T cells control these responses and ensure that they function at an appropriate level.