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INTRODUCTION

Organ transplantation is an established treatment for patients with a wide variety of end-stage diseases. It is essential for physicians to familiarize themselves with the field of transplant medicine since an encounter with a transplant candidate or recipient is inevitable.1

PHYSIOLOGY OF THE IMMUNE SYSTEM

The immune system distinguishes self from nonself to eliminate potentially harmful molecules and cells. The immune system also has the capacity to recognize and destroy abnormal cells that derive from host tissues. Any molecule capable of being recognized by the immune system is considered an antigen (Ag). The skin, cornea, and mucosa of the respiratory, gastrointestinal (GI), and genitourinary (GU) tracts form a physical barrier that is the body’s first line of defense.2

Breaching of anatomic barriers can trigger 2 types of immune response: innate and acquired. Many molecular components (eg, complement factors, cytokines, acute phase proteins) participate in both innate and acquired immunity.2

Innate immunity

Innate (natural) immunity does not require prior exposure to an Ag (ie, immunologic memory) to be effective. Thus, it can respond immediately to an invader. It recognizes mainly Ag molecules that are broadly distributed rather than specific to 1 organism or cell. Components include phagocytic cells, natural killer (NK) cells, and polymorphonuclear leukocytes. Phagocytic cells (neutrophils in blood and tissues, monocytes in blood, macrophages in tissues) ingest and destroy invading Ags. Attack by phagocytic cells can be facilitated when an Ag is coated with an antibody (Ab), which is produced as part of acquired immunity, or when complement proteins opsonize Ags.

Natural killer cells kill virus-infected cells and some tumor cells. Polymorphonuclear leukocytes (neutrophils, eosinophils, basophils, mast cells) and mononuclear cells (monocytes, macrophages) release multiple inflammatory mediators.2

Acquired immunity

Acquired (adaptive) immunity requires prior exposure to an Ag and thus takes time to develop after the initial encounter with a new invader. This system remembers past exposures and is Ag specific. Components include cell-mediated immunity from T-cell responses and humoral immunity from B-cell responses (B cells secrete Ag-specific Ab).2

B cells and T cells work together to destroy foreign Ag. Ag-presenting cells (such as dendritic cells) are needed to present Ags to T cells.2 The immune system is activated when circulating Abs or cell surface receptors recognize a foreign Ag. These receptors may be highly specific (Ab expressed on B cells or T-cell receptors) or broadly specific (such as pattern-recognition receptors called toll-like receptors). Immune activation occurs when Ab-Ag complexes or ­complement-coated molecules bind to surface receptors for the crystallizable fragment (Fc) region of immunoglobulin G (FcγR) and for C3b and iC3b.2

Once recognized, an Ag, Ag-Ab complex, or complement-molecule complex is phagocytosed. T cell–derived cytokines, particularly interferon-γ (IFN-γ), stimulate the phagocyte to produce more lytic enzymes and other bactericidal products and thus ...

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