Immunologic tolerance is defined as:
A. The removal of an antigen, or the microbes expressing the antigen, by the immune system, so that the host can tolerate the infection
B. Activation of only B cells, and not T cells, on exposure to an antigen
C. Unresponsiveness of the immune system to an antigen, which is induced by previous exposure to that antigen
D. The production of memory B cells and T cells on primary exposure to an antigen, which allows the host to tolerate a secondary exposure to the antigen
E. Vaccination of individuals against particular pathogens to prevent subsequent infections
ANS: C. Immunologic tolerance refers to the unresponsiveness of the immune system to particular antigens and develops on previous exposure to the antigen. Although the exact requirements for inducing tolerance have not been clearly defined, factors that do influence tolerance include the concentration of antigen, the mode of administration of the antigen, and the presence of costimulatory molecules on antigen-presenting cells for peptide antigens.
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