Which of the following anatomic regions is normally protected from pathogens only by humoral immune responses and not by cell-mediated immune responses?
A. Skin
B. Intestinal lumen
C. Intestinal epithelium
D. Central nervous system
E. Spleen
ANS: B. The lumen of mucosal lined tissues, such as the intestinal and bronchial lumen, are protected by IgA, which is actively secreted at these sites. T cells do not normally migrate into these lumen and are usually involved in immune responses to organisms that breach the surface linings of these structures. The epithelial linings of mucosal tissues and the skin do contain lymphocytes that protect against invading pathogens. Both antibodies and T cells are involved in responses to infections within most other tissues.
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