The principal function of the immune system is:
A. Defense against cancer
B. Repair of injured tissues
C. Defense against microbial infections
D. Prevention of inflammatory diseases
E. Protection against environmental toxins
ANS: C. The immune system has evolved in the setting of selective pressures imposed by microbial infections. Although immune responses to cancer may occur, the concept that "immunosurveillance" against cancer is a principal function of the immune system is controversial. Repair of injured tissues may be a secondary consequence of the immune responses and inflammation. Although the immune system has regulatory features that are needed to prevent excessive inflammation, prevention of inflammatory diseases is not a primary function. The immune system can protect against microbial toxins, but it generally does not offer protection against toxins of nonbiologic origin.
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