Answer:
Fever.
Step-by-step explanation:
When adaptive immune system, which involves role of B cells and T cells, fights against invading microorganisms, it leads to increase in body temperature (fever) due to various inflammatory cytokines.
Most pathogens can grow at an optimum temperature range. The higher body temperature makes an unfavorable environment for those pathogens. Due to this, most of the pathogens cannot proliferate to make their new copies and die.
Thus, the correct answer to be filled in the blank is fever.