When antigens are administered to produce immunity, why are different routes of administration considered? a. different routes allow the speed of onset of the antigen to be varied, with the intravenous route being the fastest. b. some individuals appear to be unable to respond to an antigen by a specific route, thus requiring the availability of different routes for the same antigen. c. antigen-presenting cells are highly specialized and thus require stimulation by different routes. d. each route stimulates a different lymphocyte-containing tissue, resulting in different types of cellular and humoral immunity.