Final answer:
A monocyte is a large white blood cell, ranging from 12-20 µm, without cytoplasmic granules. It differentiates into macrophages or dendritic cells to participate in immune responses, phagocytizing pathogens, and attracting other leukocytes.
Explanation :
A monocyte is a type of white blood cell that is part of the body's immune system. Originating from myeloid stem cells, monocytes are large cells, ranging from 12-20 µm in size, with distinctive indented or horseshoe-shaped nuclei. These cells are classified as agranulocytes, meaning they lack the granules present in other types of white blood cells. Monocytes circulate in the blood and lymph and can differentiate into macrophages or dendritic cells after migrating into infected tissue.
Macrophages are essential for the immune response; they phagocytize debris, foreign pathogens, and damaged cells, and release substances that attract other leukocytes to the site of infection. These cells can either be stationary in body tissues or move through the tissue fluid. They are also responsible for presenting antigens to lymphocytes, thus aiding in the adaptive immune response.