Final answer:
Macrophages are the cells that recruit and activate mononuclear cells from the blood following exposure to PPD in tuberculosis patients. They stimulate naïve T cells to become TH1 cells, which help in fighting the infection.
Step-by-step explanation:
In a tuberculosis patient, the type of cells that recruit and activate mononuclear cells from the blood following exposure to the purified protein derivative (PPD) are macrophages. These cells play a crucial role in the immune system as part of the body's defense against pathogens. Upon exposure to an infectious agent like Mycobacterium tuberculosis, macrophages can stimulate naïve T cells to become TH1 cells, which in turn secrete cytokines that bolster the macrophages' capabilities to digest and destroy the pathogens. Moreover, macrophages, along with dendritic cells and B cells, serve as important antigen-presenting cells, enhancing the body's ability to fight off infections.