Final answer:
A monocyte is the agranular leukocyte that leaves the bloodstream and becomes a wandering macrophage involved in phagocytosis.
Step-by-step explanation:
The agranulocytes are a group of leukocytes that lack the visible cytoplasmic granules seen in granulocytes. Among the agranulocytes, the monocyte is the one that, when it leaves the bloodstream, differentiates into a wandering macrophage capable of phagocytosing pathogens and debris. Unlike monocytes, neutrophils, eosinophils, and basophils are all granular leukocytes and not the ones this question refers to.
Monocytes are easily identified in the bloodstream by their large size and distinct indented or horseshoe-shaped nuclei. Upon exiting the bloodstream, these monocytes transform into macrophages which then perform critical phagocytic roles in the immune system. Some macrophages are fixed in certain tissues, while others are free to wander, seeking out and destroying pathogens and damaged cells.
A monocyte is an agranular leukocyte that leaves the bloodstream and differentiates into a wandering macrophage, which then plays a significant role in phagocytosis within the immune response.