Final answer:
Specific reactivity relates to the adaptive immune system's capacity to identify particular pathogens and remember them for a more robust response upon future exposure, exemplified by immunity to chickenpox after initial infection.
Step-by-step explanation:
The concept of "specific reactivity" in the body's response to microbial infection refers to the adaptive immune system's ability to target and remember specific pathogens. This happens through a primary response upon first exposure to a pathogen, where the system learns to recognize the pathogen and produces antibodies. Upon re-exposure, the secondary response is more rapid and potent owing to the immune memory. This is why immunity to diseases like chickenpox occurs after the first infection.