The function of the immune system is to protect the host from invasion of foreign organisms by distinguishing “self” from “non-self.” Such a system is necessary for survival. A well-functioning immune system not only protects the host from external factors such as microorganisms or toxins but also prevents and repels attacks by endogenous factors such as tumors or autoimmune phenomena. A normal immune response relies on the careful coordination of a complex network of biological factors, specialized cells, tissue, and organs necessary for the recognition of pathogens and subsequent elimination of foreign antigens. Dysfunction or deficiency of components of the immune system leads to a variety of clinical diseases of varying expression and severity, ranging from atopic disease to rheumatoid arthritis, severe combined immunodeficiency, and cancer. This chapter introduces the intricate physiology of the immune system and abnormalities that lead to diseases of hypersensitivity and immunodeficiency.
Normal Structure & Function of the Immune System
Cells of the Immune System
The immune system consists of both antigen-specific and nonspecific components that have distinct yet overlapping functions. The antibody-mediated and cell-mediated immune systems provide specificity and memory of previously encountered antigens. The nonspecific or innate defenses include epithelial barriers, mucociliary clearance, phagocytic cells, and complement proteins. Despite their lack of specificity, these components are essential because they are largely responsible for natural immunity to a vast array of environmental threats and microorganisms. Knowledge of the components and physiology of normal immunity is essential for understanding the pathophysiology of diseases of the immune system.
The major cellular components of the immune system consist of monocytes and macrophages, lymphocytes, and the family of granulocytic cells, including neutrophils, eosinophils, and basophils. Derived from hematopoietic stem cells, these fully differentiated effector cells have membrane receptors for various chemoattractants and mediators, facilitating the activation or destruction of target cells.
Mononuclear phagocytes play a central role in the immune response. Tissue macrophages are derived from blood monocytes and participate in antigen processing, tissue repair, and secretion of mediators vital to initiation of specific immune responses. These cells, abundant near mucosal surfaces that internalize microorganisms and debris, travel to secondary lymphoid organs where they process and present that antigen in a form recognizable to T lymphocytes. In addition, they function as effector cells for certain types of tumor immunity. Circulating monocytes are recruited to sites of inflammation where they mature into macrophages. Both monocytes and macrophages contain receptors for C3b (activated bound complement) and the Fc portion of both immunoglobulin G (IgG) and IgE, which facilitate the activation of these cells through antigen-specific and nonspecific immune pathways. Activation of these cells occurs both after binding to immune complexes through exposure to various cytokines and after phagocytosis of antigen or particulates such as silica and asbestos. Proteolytic enzymes and proinflammatory mediators including cytokines, arachidonic acid metabolites, ...