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 against 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: Ans. is 'a' i.e., Antigenic cross-reactivity(Ref: Ananthanarayan, 7th/e, p. 81, 82)* Antigenic cross reactivity - Sometimes single antigen stimulates production of variety of antibodies that more or less resemble the correct fit for the antigen. Each of these antibodies may attack different antigens that more or less resemble one another.