Within a very short period immunity has been placed in possession not only of a host of medical ideas of the highest importance, but also of effective means of combating a whole series of maladies of the most formidable nature in man and domestic animals.
—Elie Metchnikoff, 1905
The “maladies” Metchnikoff and the other pioneers of immunology were fighting infections and, for decades, their field was defined in terms of the immune response to infection. We now understand that the immune system is as much a part of everyday human biologic function as the cardiovascular or renal systems. In its adaptive and disordered states, infectious diseases are only one of the major player along with cancer and autoimmune diseases. Students of medicine take up immunology as a separate unit with its own text covering the field broadly. This chapter is not intended to fulfill that function, or to be a shortened but comprehensive version of those sources. It is included as an overview of aspects related to infection for other students and as an internal reference for topics that reappear in later pages of this book. These include some of the greatest successes of medical science. The early and continuing development of vaccines that prevent and potentially eliminate diseases is but one example. In addition, knowledge of the immune response to infection is integral to understanding the pathogenesis of infectious diseases. It turns out that one of the main attributes of a successful pathogen is evading or confounding the immune system.
The immune response to infection is presented as two major components—innate immunity and adaptive immunity. The primary effectors of both are cells that are part of the white blood cell series derived from hematopoietic stem cells in the bone marrow (Figure 2–1). Innate immunity includes the role of physical, cellular, and chemical systems that are in place and that respond to all aspects of foreignness. These include mucosal barriers, phagocytic cells, and the action of circulating glycoproteins such as complement. The adaptive side is sometimes called specific immunity because it has the ability to develop new responses that are highly specific to molecular components of infectious agents, called antigens. These encounters trigger the development of new cellular responses and production of circulating antibody, which have a component of memory if the invader returns. Artificially creating this memory is, of course, the goal of vaccines.
Human blood cells. Stem cells in the bone marrow divide to form two blood cell lineages: (1) the lymphoid stem cell gives rise to B cells that become antibody-secreting plasma cells, T cells that become activated T cells, and natural killer cells. (2) The common myeloid progenitor cell gives rise to granulocytes and monocytes that give rise to macrophages and dendritic cells. (Reproduced with permission from Willey JM: Prescott, Harley, & Klein’s Microbiology, 7th edition. McGraw-Hill, 2008.)