Final answer:
Monocytes are the largest leukocytes in an animal's circulation and are crucial for the immune system, turning into macrophages to remove pathogens and debris.
Step-by-step explanation:
The largest leukocytes in an animal's circulation are monocytes. Monocytes are essential to the immune system, as they differentiate into macrophages after leaving the bloodstream, and they phagocytize pathogens, dead cells, and cellular debris. Unlike erythrocytes, which are numerous but lack a nucleus and organelles, leukocytes like monocytes are complete cells with a nucleus and organelles and are larger than erythrocytes. Leukocytes fulfill a variety of functions, such as defending the body against invading microorganisms and mutated body cells and facilitating the inflammatory response along with other leukocytes like eosinophils and basophils.