Final answer:
Monocytes are characterized by their distinct, indented or horseshoe-shaped nucleus and are a type of agranulocytes. They differentiate into macrophages with high phagocytic activity, which is crucial for the immune response.
Step-by-step explanation:
A distinguishing characteristic of monocytes, which are a type of white blood cell, is their distinct nucleus shape. Unlike granulocytes such as neutrophils, eosinophils, and basophils that possess a lobed nucleus and visible granules in their cytoplasm, monocytes fall under the category of agranulocytes. They typically have a large size (12-20 µm in diameter) with an indented or horseshoe-shaped nucleus. Monocytes are versatile in their function; they differentiate into macrophages and dendritic cells, which are highly active phagocytes participating in the immune response by engulfing and digesting pathogens and debris. This high phagocytic activity along with the secretion of antimicrobial and chemical signals is crucial in the body's defense mechanisms. Some macrophages remain fixed in tissues, while others roam the body, responding to infection or injury.