This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Immunology, Subject name: Microbiology
Which of the following is characteristic of the mucosal immune system? 
Select and explain. 
A. A vigorous response is made to all nonself antigens encountered, B. Chronic inflammation makes an inhospitable environment for microbes, C. IL-2 and IFN-g contribute to a Th-1 like environment, D. Tolerance to foreign antigens is the norm rather than the exception
D: Tolerance to foreign antigens is the norm rather than the exception
Answer: D: Tolerance to foreign antigens is the norm rather than the exception 
Explanation: Ans. (d)Because the mucosal immune system is constantly exposed to so many nonself epitopes that are essentially harmless, it is tolerant of most of them. Although it can respond to microbes that pose a pathogenic threat, the mucosal system generally avoids the development of chronic inflammation because of the damage that could be inflicted on the delicate mucosal linings. The immunologic environment is generally described as more Th - 2 like than Th - 1 like. IgG is present at far lower levels than is IgA.