A)the human body's ability to distinguish self from nonself. B)the observation that some strains of the pathogen that causes dengue fever cause worse disease than others. C)the ability of a helper T cell to signal B cells via cytokines. D)the ancient observation that someone who had recovered from the plague could safely care for those newly diseased. E)the ability of the immune system to present antigen fragments in association with MHC antigens.