Ian R Tizard - Comparative Mammalian Immunology: The Evolution and Diversity of the Immune Systems of Mammals

1. Tables in Chapter 6
2. Tables in Chapter 8
3. Tables in Chapter 9
4. Tables in Chapter 12
5. Tables in Chapter 13
6. Tables in Chapter 14
7. Tables in Chapter 17
8. Tables in Chapter 19
9. Tables in Chapter 21
10. Tables in Chapter 25
11. Tables in Chapter 26

1. Figures in Chapter 1
2. Figures in Chapter 2
3. Figures in Chapter 3
4. Figures in Chapter 4
5. Figures in Chapter 5
6. Figures in Chapter 6
7. Figures in Chapter 7
8. Figures in Chapter 8
9. Figures in Chapter 9
10. Figures in Chapter 10
11. Figures in Chapter 11
12. Figures in Chapter 12
13. Figures in Chapter 13
14. Figures in Chapter 14
15. Figures in Chapter 15
16. Figures in Chapter 16
17. Figures in Chapter 17
18. Figures in Chapter 18
19. Figures in Chapter 19
20. Figures in Chapter 20
21. Figures in Chapter 21
22. Figures in Chapter 22
23. Figures in Chapter 23
24. Figures in Chapter 24
25. Figures in Chapter 25
26. Figures in Chapter 26

Ian R. Tizard, BVMS, BSc, PhD, ScD (Hons), ACVM (Hons)

Department of Veterinary Pathobiology, Texas A&M University, TX, United States

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To Claire

Ian R. Tizard

Over many years of teaching veterinary immunology, I often wondered about the seemingly random differences between the immune systems of domestic mammals. We have known about the immune peculiarities of individual domestic mammals for many years. Pigs have inverted lymph nodes. Cattle have unusual immunoglobulin variable regions. Sheep have 13 IgA subclasses. Horses have seven IgG subclasses. This has always seemed to be very untidy. One of my motivations for writing this text was to bring order to what seemed to be a chaotic situation. Thus, in looking at mammals as a whole, I had hoped to identify some phylogenetic pattern behind their immunologic diversity. That was not to be. The diversity of infectious and parasitic diseases as well as the enormous diversity of the mammals themselves has ensured that immune systems have to be diverse down to the species level. Likewise, the major antigen receptor systems, the major histocompatibility complex, and the antigen receptors on T and B cells have evolved as a result of an essentially random life and death process. The evolution of the mammalian immune systems has been empirical and opportunistic. They use whatever works.

Another feature that emerged during the writing of this book is its focus on receptors. On reflection this is unsurprising. Immune responses require extensive conversations between multiple cell types as well as foreign antigens. These conversations are of necessity, conducted through ligand-receptor binding. Receptors thus control the immune system by controlling cell interactions. The language of these interactions is determined by receptors and the genes that encode them. Phylogeny thus controls the language and hence the conversation.

This is a book about differences. We know that all mammals are warm-blooded. They suckle their young and have hair. Equally importantly, we recognize that mammals come in an enormous diversity of shapes and sizes. The same applies to their immune systems. Just as mammals diversified in response to environmental and other selective pressures, so too did their immune systems. While all conform to a similar basic pattern, its the differences that provide important information and interesting twists to the plot. Thus, this text summarizes the common mechanisms and pathways of the innate and adaptive immune systems. It also seeks to focus on the differences and peculiarities of each mammalian order and, if warranted, the unique features of each species.

One feature of modern immunology as reflected in the literature is its use of a single flagship species to represent an entire order. This focus is not confined to humans and laboratory rodents. Other orders have their favorite species as well. Thus the domestic dog is the clear leader in caniforms, the bottlenose dolphin in cetaceans, and the black flying fox (Pteropus alceto) among chiropterans. However, it must be remembered that while these species may be common and convenient to study, they may not, in fact, be typical of the other members of their orders.

Every living creature is obliged to invest in its defenses against microbial invasion. Every mammal that has existed over the past 300 million years lived and died in the presence of bacteria, viruses, and diverse parasites. On the other hand, microbes and mammals have also developed mutually beneficial commensal relationships. Microbes will take every opportunity to exploit animals for shelter and nutrition. Mammals are obliged to respond by investing in their defenses. Immune defense systems directed against micropredators are mandatory. Mammals have to defend not only against many different bacteria, viruses, and parasites but also against a diversity of threats of different frequencies and significance. From the skin and environmental bacteria that can infect cuts and scratches to the highly lethal pandemic agents that can eliminate whole populations. Immunity incurs costs and mammals must balance both the costs and benefits. This balance will, of necessity, be different for every species and be profoundly influenced by environmental, behavioral, and age issues.