Overview Of The Immune System

Although a more detailed description of the immune system is provided in other articles of this encyclopedia, it is necessary to briefly review the basics of immunology. This is not meant to be a comprehensive review but, rather, is meant to provide the reader with sufficient background to interpret research in the field of behavioral neuroimmunology.

The general purpose of the immune system is to defend the body against infection and disease by identifying and eliminating foreign "nonself" pathogens and mutated "self" cells. This is accomplished through the activity of two general categories of immunity: innate (also called nonspecific or natural) and acquired (also called specific) immunity. Natural killer (NK) cells, macrophages, and neutrophils are agents of innate immunity. These cells serve a surveillance function and will attack pathogens or cancerous cells without specificity and without requiring prior exposure to the invader. In contrast, acquired immunity involves a specific cascade of reactions to a particular antigen, and the magnitude of response to this pathogen increases with each successive exposure. Acquired immune responses can be classified into two types: cell-mediated and humoral immunity. Cellmediated immunity is carried out by T lymphocytes (which mature in the thymus), whereas humoral immunity is mediated by antibodies that are produced by B lymphocytes (which mature in the bone marrow in humans).

Perhaps the most efficient way of reviewing the acquired immune response is to trace how the immune system would respond to a given pathogen (Fig. 1). Upon initial infection, a macrophage or other antigen-presenting cell (APC) encounters the pathogen, engulfs it, and digests it. The macrophage then displays

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the foreign antigen on its surface so that it can be recognized by antigen-specific T cells. In addition, the macrophage secretes cytokines or chemical messengers such as interleukin-1 (IL-1) that stimulate helper T (CD4+) cells to begin proliferating. Upon encountering the APC, helper T cells secrete IL-2 and interferon (IFN), which stimulate cytotoxic T cells (CD8+) to destroy infected cells. In addition, helper T cells secrete IL-4, which activates the humoral branch of the immune response. IL-4 stimulates B cells either to mature into plasma cells or to become B memory cells. Plasma cells produce large quantities of antibody specific to the antigen that serve to destroy it. B memory cells "remember" the particular antigen so that, upon subsequent reexposure, the speed and magnitude of the immune response will be increased.

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