Final answer:
Active phagocytes include neutrophils and monocytes, with neutrophils being the first to respond to infection and monocytes developing into macrophages that aid in both cleaning and immune response activation.
Step-by-step explanation:
The leukocytes that are active phagocytes are neutrophils and monocytes. Neutrophils are the most numerous type of phagocytes, making up at least half of the total circulating leukocytes. They are crucial to the body's innate immune response and often the first cells to arrive at the site of an infection. Monocytes give rise to macrophages, which are highly efficient phagocytes that can engulf a significant number of pathogens or other cells. Macrophages not only clean up dead and damaged cells but also play a key role in antigen presentation, thus activating the adaptive immune system.