18. Introduction

Despite relatively constant exposure to pathogenic microbes in the environment, humans do not generally suffer from constant infection or disease. Under most circumstances, the body is able to defend itself from the threat of infection thanks to a complex immune system designed to repel, kill, and expel disease-causing invaders. Immunity as a whole can be described as two interrelated parts: nonspecific innate immunity, which is the subject of this chapter, and specific adaptive host defences, which are discussed in the next chapter.

The nonspecific innate immune response provides a first line of defence that can often prevent infections from gaining a solid foothold in the body. These defences are described as nonspecific because they do not target any specific pathogen; rather, they defend against a wide range of potential pathogens. They are called innate because they are built-in mechanisms of the human organism. Unlike the specific adaptive defences, they are not acquired over time and they have no “memory” (they do not improve after repeated exposures to specific pathogens).

Broadly speaking, nonspecific innate defences provide an immediate (or very rapid) response against potential pathogens. However, these responses are neither perfect nor impenetrable. They can be circumvented by pathogens on occasion, and sometimes they can even cause damage to the body, contributing to the signs and symptoms of infection (Figure 1).