Final answer:
Macrophages differentiate from circulating blood monocytes. Monocytes originate from myeloid stem cells and become macrophages once they leave the circulation. Macrophages play a crucial role in phagocytosis and releasing chemicals to attract other leukocytes to the site of infection.
Step-by-step explanation:
Macrophages differentiate from circulating blood monocytes. Monocytes originate from myeloid stem cells and are typically recognized by their large size and indented nuclei. Once they leave the circulation, monocytes become macrophages and phagocytize debris, pathogens, and damaged cells. Macrophages also release chemicals that attract other leukocytes to the site of infection.