INTRODUCTION

The immune system exists to combat infection. By convention, there are considered to be two ways that the immune system combats infection. These are represented as the two poles of the horizontal axis in Fig. 3.1. The innate immune system – in effect the inflammatory response – is the non-specific component which responds indiscriminately to infection and other forms of injury and which uses phagocytosis as a major effector route. The adaptive immune system is specific and distinguishes not only between infection and injury but also between different types of infection. This system uses antibody and T-cell receptors for recognition. The fine specificity of the adaptive reaction, and how it is controlled, is one of the most remarkable of all biological processes and incorporates both memory – recall of past experience – and tolerance – recall to not respond.

From an immunologist's perspective, the ability of an infective agent to survive this double jeopardy immune response represents failure. But organisms can – and do – survive the response all the time. They do this by interfering with different stages in the natural history of the response, and six major examples of how this may happen are illustrated in Fig. 3.1 and Plate I. Figure 3.1 and Plate I also highlight the importance of location: fortunately most organisms are encountered at sites of immune surveillance; but, if they do happen to localise to a site of immune privilege, this may carry with it an intrinsic survival advantage.