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Together with a series of physical barriers (the skin, mucous membranes, tears, and stomach acid), the immune system helps to defend the body against foreign or harmful substances that attempt to infiltrate it (Table 12-1). Such substances include microorganisms (bacteria, viruses, and fungi), parasites (such as worms), cancer cells, and even transplanted organs and tissues. Antigens are entities within or on bacteria, viruses, other microorganisms, or cancer cells that stimulate an immune response in the body. Antigens may also exist independently—eg, as pollen or food molecules. A normal immune response consists of the initial recognition of a foreign antigen, and the mobilization of forces to defend against it.

Table 12-1. Terms and Definitions of the Immune Systema

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