This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Immunology, Subject name: Microbiology
[{'content': 'The protection againts small pox by previous infection with cowpox represents - \nSelect and explain. \nA. Antigenic cross-reactivity, B. Antigenic specificity, C. Passive immunity, D. Innate immunity', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Cross-reactivity applies to the reaction between two different species as opposed to the self-reactivity.In immunology, the cross-reactivity has a more narrow meaning of the reaction between an antibody and an antigen that differs from the immunogen.', 'role': 'assistant'}]