INTRODUCTION

All organisms, including humans, live in a sea of infectious or toxic agents inhabiting the air we breathe, the food we eat, and the water we drink. Yet most of us and other organisms are healthy for the majority of our lives because we can resist these agents.

The functions of a host immune system are complex and often overlap. One function, of course, is host defense—to protect the body from infectious agents. Another function is surveillance; cells of the immune system con­stantly find and destroy mutant (cancer) cells that continually arise in hosts. A third function is homeostasis, in which damaged or worn out parts of the body are removed to allow for replacement by new tissues.

To protect hosts from infectious diseases and cancers, the immune system must be able to rapidly distinguish between cells and molecules associated with the host's own body (termed “self”) from cells and molecules not considered part of the host body (“non­self”) (Clough and Roth 1998). The resistance by a host against infectious and toxic agents generally has been classified into two broad but overlapping physiological systems: the innate system, also called the nonspecific or natural system, and the acquired, or specific, immune system.

The innate system gives a general protection and is continually in operation, or at least ready at all times to respond against foreign threats. The innate system has a fairly low level of dis­crimination, and essentially distinguishes “self” from “nonself” (Wakelin and Apanius 1997).

The acquired immune system is activated by, and responds to, specific foreign chemicals of invading parasites or toxins, or to unusual cells such as cancer cells or parasite-infected cells. The term antigen is used to denote any chemical recognized as foreign (nonself) to the host and that stimulates an immune response or reacts with products of the immune response. The acquired immune system is capable of sophisticated discrimination and can distinguish among an almost infinite number of foreign antigens (Wakelin and Apanius 1997). This acquired system also is slow to respond and initially may require one to two weeks for an effective response the first time it encounters an antigen; however, it typically responds more quickly on subsequent exposure to the same antigen.

A brief description of these major systems is provided in this chapter. Greater depth on these topics is available from a number of resources (Sell 1996; Janeway and Travers 1997; Wakelin and Apanius 1997; Clough and Roth 1998; Tizard 2002, 2004; Davison et al. 2008; Parnham 2009; Tizard 2013).