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The function of the immune system is to protect the host from invasion of foreign organisms by distinguishing “self” from “non-self.” 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 and participates in tissue repair. A normal immune response relies on the careful coordination of a complex network of biological factors, specialized cells, tissues, 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 autoimmune disease, primary immunodeficiency, and cancer. This chapter introduces the intricate physiology of the immune system and abnormalities that lead to diseases of hypersensitivity and immunodeficiency.

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 are adaptive, providing specificity and memory of previously encountered antigens. The nonspecific or innate defenses include epithelial barriers, mucociliary clearance, phagocytes, dendritic cells, innate lymphoid cells, mast cells, and complement proteins. Despite being phylogenetically primitive and lacking 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.



Cells 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, basophils, and mast cells. Derived from hematopoietic stem cells, these fully differentiated effector cells have membrane receptors for various chemoattractants, immunoglobulins, soluble mediators, and cell-surface proteins, which facilitate cellular homing, the activation or destruction of target cells. Furthermore, it is increasingly recognized that many immune cell-types have subsets, with distinct cytokine profiles and surface markers.

Mononuclear phagocytes play a central role in the immune response. Tissue macrophages are derived from blood monocytes and participate in antigen processing and secretion of mediators vital to initiating specific immune responses. Abundant near mucosal surfaces and recruited to sites of inflammation, these cells phagocytose and internalize microorganisms and debris, then travel to secondary lymphoid organs where they process and present that antigen in a form recognizable to T lymphocytes. In addition, macrophages function as effector cells for certain types of tumor immunity and participate in tissue repair through promotion of angiogenesis and fibrosis.

Macrophages are activated by binding of extracellular molecules to surface-bound receptors. Receptors for complement component C3b (bound fragments of complement, activated by microbes and antigen-bound immunoglobulins; ie, immune complexes) and the Fc portion of both ...

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