As for most things in life, turning things off in the immune system is as important as turning them on. Biologists usually like to focus on the positive, the ways in which natural processes get started. However, this is not the case for the immune system. Numerous studies have addressed the problems of resolving immune responses, perhaps because inhibition and resolution of the immune response are so important, required, as they are, to prevent autoimmunity and reduce ongoing inflammation.

The body responds to infection by inducing many processes. Often granulocytes are the first cells to appear at sites of infection. Later, antigen and danger signals from the infected area arrive in draining lymph nodes, either free from cellular carriers or on migrating dendritic cells. Together these materials activate T and B cells in the nodes, causing huge expansions in the numbers of antigen-specific lymphocytes and some increases in the numbers of nonspecific cells, carried along, by various means, for the ride. These cells migrate to the site of infection, where they either directly or indirectly, by recruiting and activating other cell types, cause the destruction, elimination, or inactivation of the invading organism. After the invader has been disposed of, however, various processes that eliminate the recruited cells are brought into play. Also, there is considerable evidence that even during the acute phase of the immune response, various feedback inhibitory pathways are induced, making sure that the huge cellular expansion and cytokine storm that accompany the immune response do not overwhelm the host. With all this in mind, it is clear that immune responses are controlled and resolved in many ways. Discussions of the results of many of these studies are included in this volume. While these studies are valuable and help us to some extent understand how immunity is controlled, it is still apparent and a major problem in the field that we do not know which of the many processes dominates in different circumstances and even why the immune response is controlled in so many ways. Some of these issues are discussed in this introduction.