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
True about active immunity 
Select and explain. 
A. Less effective, B. Can be given in immunodeficient state, C. Immunological memory present, D. No lag period
C: Immunological memory present
Answer: C: Immunological memory present 
Explanation: When B cells and T cells are activated by a pathogen, memory B-cells and T- cells develop, and the primary immune response results. Throughout the lifetime of an animal these memory cells will "remember" each specific pathogen encountered, and are able to mount a strong secondary response, if the pathogen is detected again. Ref: Ananthanarayan & Panikers textbook of microbiology 9th edition pg.no. : 81-84