Final answer:
Option (B), In a DTH reaction, activated macrophages are the primary cells responsible for recruiting mononuclear cells to the area of antigen exposure.
Step-by-step explanation:
In a delayed-type hypersensitivity (DTH) reaction to common antigens, the primary cells responsible for recruiting mononuclear cells to the area are the activated macrophages. These cells play a significant role following the sensitization step where antigens are introduced into the skin.
There, they are phagocytized by local antigen presenting cells, which then activate helper T cells leading to the differentiation into memory TH1 cells. Upon re-exposure to the antigen, memory TH1 cells release cytokines that activate macrophages, and these macrophages are principally accountable for the recruitment of mononuclear cells and subsequent tissue damage, as observed in examples like the Mantoux skin test and contact dermatitis.