are a central part of the mammalian innate immune response, appear to be involved primarily in development of the insect or in feeding behavior. Only one Toll receptor appears to play a role in defending the insect against fungal infections. Nonetheless, it is intriguing to see that parallels of what we know as the mammalian innate and adaptive defense systems are so widespread in other creatures. Interest in the evolution of the mammalian immune system is a relatively new area of research focus, so there are likely to be many more insights in the future as this research progresses.
The Dark Side of the Adaptive Defenses: Autoimmune Disease
Complex figures such as those shown in this chapter illustrate the manifold interactions that occur between cells of the immune system when they activate each other. All of those proteins labeled CD followed by a number have an important role. They help ascertain that the contacts between the immune cells are occurring in response to true foreign antigen presentation events and not accidental associations between cells of the immune system and other cells in the body. They serve as insurance that the immune system will not go out of control and attack self cells.
Unfortunately, this safeguard is sometimes breached, like when bacterial antigens contain epitopes that mimic human antigens. In such cases, the adaptive defenses can produce antibodies or CTLs that mount an attack against the host, thereby creating a set of conditions that can lead to autoimmune disease. If the targeted tissue is the heart or another vital organ, the misguided attack can be lethal. In later chapters, we will encounter examples of autoimmunity initiated by the adaptive immune defenses against bacterial components that mimic host antigens. Bacteria that elicit an autoimmune response are particularly difficult targets for vaccine development. Such a vaccine could induce an autoimmune response when the vaccinated person is exposed to the bacterium, which could make the disease worse rather than preventing it. Thus, one of the concerns of vaccine producers is to make sure that their vaccine does not elicit an autoimmune response.
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