Final answer:
Monocytes are the leukocytes that differentiate into macrophages, which play a crucial role in the immune system by phagocytizing pathogens and releasing antimicrobial substances.
Step-by-step explanation:
Leukocytes that differentiate into macrophages are monocytes. Monocytes are a type of white blood cell that originate from myeloid stem cells and usually make up about 2-8 percent of the total leukocyte count. These cells are identifiable by their large size and distinctive indented or horseshoe-shaped nuclei.
Upon leaving the bloodstream and entering tissue, monocytes transform into macrophages, which then become active in phagocytizing various types of debris, including foreign pathogens and dead cells. Furthermore, macrophages release chemicals such as antimicrobial defensins and chemotactic agents to attract other immune cells to sites of infection and contribute to the body's defense mechanisms.