The avian monocyte is the largest circulating blood cell. Monocytes, like lymphocytes, are considered mononuclear cells possessing a single, large, rather round nucleus. Monocytes closely resemble large lymphocytes in size and shape. The nucleus of these cells is large with a irregular indentation. There is more cytoplasm associated with the monocyte than the large lymphocyte and vacuoles are often present in the cytoplasm.
Monocytes are motile cells which can migrate using ameboid movements.
Monocytes are also phagocytic. Monocytes tend to have a relatively short lifespan within the circulation. The reason for this is their tendency to migrate through the vascular endothelium and enter the tissues. There they become scavengers known as tissue macrophages.