ABSTRACT
In this chapter, we begin to examine the ways in which a host defends itself against any foreign material that enters the body. Materials that are seen by the host as foreign are called antigens. An antigen is any substance that can elicit an immune response in a host. During this discussion, it is important that you keep in mind what you learned in Chapters 5 and 6 about compromised hosts and about the mechanisms that pathogens use to undermine our defenses. The interaction between pathogen and host is one of the cornerstones of infectious disease. We will see in this chapter that there are several natural barriers to infection. There are also highly effective and lethal nonspecific defense mechanisms that hosts can employ as guards against infection. Along with the specific immune reactions that we will learn about in the next chapter, these nonspecific defense mechanisms help to protect us against a variety of potentially dangerous infections.
