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
The protection againts small pox by previous infection with cowpox represents - 
Select and explain. 
A. Antigenic cross-reactivity, B. Antigenic specificity, C. Passive immunity, D. Innate immunity
A: Antigenic cross-reactivity
Answer: A: Antigenic cross-reactivity 
Explanation: 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.