This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Microbiology
The exact part of the antigen that reacts with the immune system is called as - 
Select and explain. 
A. Clone, B. Epitope, C. Idiotope, D. Effector
B: Epitope
Answer: B: Epitope 
Explanation: None