What is the largest leukocyte seen on a normal peripheral blood smear?

The largest leukocyte observed on a normal peripheral blood smear is the monocyte. Monocytes are characterized by their large size, often exhibiting a kidney-shaped nucleus and abundant cytoplasm. They typically range from 12 to 20 micrometers in diameter, significantly larger than other types of leukocytes such as lymphocytes, neutrophils, and basophils.

Monocytes play a crucial role in the immune system, functioning as precursors to macrophages and dendritic cells. Their larger size enables them to engulf pathogens and debris more effectively, facilitating the immune response. In addition to their size, the distinct morphology of monocytes on blood smears, including a pale blue cytoplasm and a variegated nuclear appearance, further assists in their identification.

Other leukocytes, like lymphocytes and neutrophils, are smaller in size and have different roles in immune function. Lymphocytes, for example, are primarily involved in adaptive immunity, while neutrophils are frontline defenders against infection. Basophils, although also present in the blood, are the least abundant and their size does not rival that of monocytes. Thus, the identification of monocytes as the largest leukocyte is consistent with their physical characteristics and biological functions.