j 5 1 The Immune System 1.1 Introduction (Section 1.4). We then introduce the major classes of molecule involved in functions such as the detection of infection, the All living things – animals, plants and even bacteria – can act as recruitment of cells to infected sites, communication between hosts for infectious organisms and thus have evolved mechan- cells and tissues, signalling within cells, and, usually, elimi- isms to defend themselves against infection. Infection can be nation of the infectious agent (Section 1.5). by other living things, non-living things (viruses) and possibly The immune system that humans have evolved is, however, even molecules (prions). Since it is so crucial to our own not perfect and we discuss some of these imperfections at the survival, much of our understanding of immunity has come end of this chapter (Section 1.6). The immune system is a very from studies in humans – particularly in relation to the causes effective killing machine, and if it goes wrong it can cause and prevention of disease – but deep insights have also come severe disease and even death of its host. To cover these latter fi from experimental studies in animals such as mice. For these areas we rst consider how the immune system is able to reasons, in this book we concentrate on the immune systems discriminate between what needs to be eliminated and what – of humans and mice. These, along with other more recently does not particularly in the case of adaptive immunity, which evolved organisms (e.g. birds and amphibians), have the most has evolved to recognize molecular structures largely at ran- complex and sophisticated immune systems, but the origins dom. We then introduce the different ways in which the of these can in many instances be traced back to the most immune system can cause damage if it becomes directed not distant and ancient species in evolutionary history. to infectious agents, but to otherwise harmless targets, includ- In this chapter we provide an overview of immunology in ing many inert substances around us and within the tissues of which we introduce the key players in immunity, largely the host itself. We discuss the problems of transplants (some focussing on the immune systems of humans and mice. We of which can even attack their hosts) and why the immune start by briefly considering how infection can be sensed by the system fails to reject malignant tumours (cancer). Finally, we host organism and how it is possible for a host to recognize turn from problems to solutions and introduce two areas in many very different infectious agents (Section 1.2). We then which either the intact immune system and components of fi introduce the tissues and specialized organs where immune immunity can be harnessed for our own bene t and from responses occur (Section 1.3). Toeliminate different infections which tools can be derived to treat disease. effectively, immune responses need to be tailored to particular By the end of this chapter you should have insight into of types of infection. This requires a variety of cells and molecules the basic properties and functions of the immune system, that can interact coherently to generate the mechanisms that and will understand the principles of its roles in defence are needed to eliminate each type of infection. As these against infectious disease. You will start to have an appre- mechanisms help to bring about or effect the elimination ciation of why it is pivotal to life, disease and death, and how of infectious agents they are termed effector mechanisms. it is important not only in prevention of disease but in its Defence against infection is divided into two main forms causation. This chapter will lead you on to the following termed innate immunity and adaptive immunity. Innate chapters where different areas of immunity are discussed defence mechanisms are present in different forms in all in greater depth. multi-cellular organisms, including plants. Adaptive defence mechanisms have evolved more recently in vertebrates. In 1.2 vertebrates, the interaction of innate and adaptive immune mechanisms is essential for the generation of effective immu- Host Defence Against Infection nity to infection. To introduce the mechanisms of immunity we start by We need immune responses to defend ourselves against describing the different types of immune cells and their infection. Many different kinds of organism have the potential function in innate and adaptive immunity to infection to infect us and, if they do so, can cause us harm in many Exploring Immunology: Concepts and Evidence, First Edition. Gordon MacPherson and Jon Austyn. Ó 2012 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2012 by Wiley-VCH Verlag GmbH & Co. KGaA. 6 j 1 The Immune System different ways. To deal with all these potential threats we, defence. In mammals, defence mechanisms can be passive or as hosts for infectious agents, need a variety of different kinds active. Passive defence comes in the form of natural barriers of host defence mechanisms. Indeed this applies for any that hinder infection. Examples are skin, which prevents living organism. access of microbes to the underlying tissue, and gastric acid in the stomach which, not surprisingly, can kill many 1.2.1 microbes that might be ingested with food. Their existence Infectious Agents is quite independent of the presence of infection. Active defence is brought about by immune responses that involve To understand how the immune system works in infection we a diversity of different effector mechanisms that are induced need to know who the aggressors are. Potentially infectious by the presence of infection and which may eliminate the agents include the following: microbe. Thus, all forms of active immunity depend on specific recognition of molecules present in the infecting . Viruses, which are non-living entities. Common examples agent. This is turn leads to a response, involving are influenza virus, human immunodeficiency virus (HIV) the interaction of cells and molecules to produce different and herpes simplex virus (HSV,which can cause cold sores effector mechanisms that can often eliminate the infection. or genital ulcers). Immunity is itself divided into two different forms – innate . Bacteria, are single-celled prokaryotic organisms. Exam- and adaptive. Innate responses occur rapidly and can generate ples include Staphylococcus and Streptococcus that cause effector mechanisms that are effective within minutes or acute infections such as abscesses and sore throats, and hours of infection. In contrast, adaptive immunity takes Mycobacteria that cause chronic infections such as tuber- much longer to become effective, usually over a few days. culosis and leprosy. In immunity to most forms of infection, however, both innate . Fungi, which are unicellular, such as Candida that causes and adaptive immunity are essential. A major advantage of thrush, or multicellular. adaptive immune responses, not seen with innate immunity, . Parasites, which are eukaryotic organisms. Some are sin- is that they generate memory – a second infection with the gle-celled protozoa that cause diseases such as malaria, same microbe elicits a stronger, faster and usually more others are large, multicellular organisms (metazoa) such as effective response. See Figure 1.1. tapeworms. In this book, for convenience, we will sometimes refer to 1.2.3 smaller infectious agents, including viruses, as microbes Immune Recognition because they are microscopic in size. However, many para- sites, the metazoa, are often far from microscopic in size. Different types of cells and molecules are involved in the initiation of innate and adaptive immune responses although, 1.2.2 as mentioned above, their interaction is essential in defence Host Defence against most infectious agents. So what do the innate and adaptive arms of immunity do in general terms? Broadly All organisms possess mechanisms to defend themselves speaking we can view some components of the innate against infection, and immunity is a specialized form of host immune system as being involved in the detection of Fig. 1.1 Mechanisms of defence against infection. Natural barriers. These stop infectious agents entering the host or provide a hostile environment. Physical barriers to infection include the epithelia of the skin, lung and airways, and the gastro-intestinal and urogenital tracts. Cells in these barriers may also secrete agents that kill infectious agents. Innate immunity. This is the first form of immunity induced by infectious agents. Cells and molecules such as phagocytes and complement can make rapid responses that may eradicate the infection. Adaptive immunity. Later adaptive responses may be generated if the infectious agent is not killed by innate immunity. Cells and molecules such as lymphocytes and antibodies take longer to become effective, but adaptive immunity can also lead to a state of long-lasting resistance to re-infection termed immunological memory (not shown). 1.2 Host Defence Against Infection j 7 harmful things that represent danger to the organism, 1.2.3.1 Recognition in Innate Immunity: Pattern Recognition such as general classes of microbes that may have infected the Receptors host. Other components then endeavour to eliminate the The key components of the innate immune system include microbe. In contrast, the adaptive immune system can dis- cells such as phagocytes and soluble molecules such as criminate very precisely between individual microbes, even of complement. These work together to sense the presence of the same type, but can generally only make a response if it has infection. The recognition of potentially dangerous microbes been informed by the innate system that what is being usually leads to the generation of inflammation, familiar to us recognized is dangerous. If so, adaptive responses may then all. One way of viewing this is that the innate immune systems help to eliminate the microbe, if it has not already been of multi-cellular organisms can generate alarm signals in eradicated during the earlier innate response.