Passive immunity is the transfer of active humoral immunity in the form of ready-made antibodies, from one individual to another. Passive immunity can occur naturally when maternal antibodies are transferred to the fetus through the placenta or can be induced artificially, when high levels of human antibodies specific for a pathogen or toxin are transferred to non-immune individuals. Emil Adolf von Behring and Kitasato Shibasaburo discovered the antitoxin-based immunity to diphtheria and tetanus and antitoxin became the first major success of modern therapeutic immunology.
So, the correct answer is option D.
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