White blood cells (leukocytes). Monocytes
Monocytes are the largest cells of the blood (averaging
15–18 micrometres), and they make up on the average
about 7 percent of the leukocytes. The nucleus is relatively
big and tends to be indented or folded rather than multilobed.
The cytoplasm contains large numbers of finegranules,
which often appear to be more numerous near the cell
membrane. Monocytes are actively motile and phagocytic.
They are capable of ingesting infectious agents as well
as red cells and other large particles, but they cannot
replace the function of the neutrophils in the removal
and destruction of bacteria. Monocytes usually enter
areas of inflamed tissue later than the granulocytes.
Often they are found at sites of chronic infections.
In the bone marrow, granulocytes and monocytes arise
from a common precursor under the influence of the granulocyte–macrophage
colony-stimulating factor Monocytes leave the bone marrow
and circulate in the blood. After a period of hours
the monocytes enter the tissues, where they develop
into macrophages, the tissue phagocytes that constitute
the reticuloendothelial system (or macrophage system).
Macrophages occur in almost all tissues of the body:
those in the liver are called Kupffer cells; those in
the skin are called Langerhans cells. Apart from their
role as scavengers (see video), macrophages play a key
role in immunity by ingesting antigens and processing
them so that they can be recognized as foreign substances
by lymphocytes.
