Life Span of the White Blood Cells

Genesis of the White Blood Cells
Early differentiation of the pluripotential hematopoietic
stem cell into the different types of committed
stem cells is shown in Figure 32–2 in the previous
chapter. Aside from those cells committed to form red
blood cells, two major lineages of white blood cells are
formed, the myelocytic and the lymphocytic lineages.
The left side of Figure 33–1 shows the myelocytic
lineage, beginning with the myeloblast; the right
shows the lymphocytic lineage, beginning with the
lymphoblast.
Life Span of the White Blood Cells
The life of the granulocytes after being released from
the bone marrow is normally 4 to 8 hours circulating
in the blood and another 4 to 5 days in tissues where
they are needed. In times of serious tissue infection,
this total life span is often shortened to only a few
hours because the granulocytes proceed even more
rapidly to the infected area, perform their functions,
and, in the process, are themselves destroyed.
The monocytes also have a short transit time, 10 to
20 hours in the blood, before wandering through the
capillary membranes into the tissues. Once in the
tissues, they swell to much larger sizes to become tissue
macrophages, and, in this form, can live for months
unless destroyed while performing phagocytic functions.
These tissue macrophages are the basis of the
tissue macrophage system, discussed in greater detail
later, which provides continuing defense against
infection.
Lymphocytes enter the circulatory system continually,
along with drainage of lymph from the lymph
nodes and other lymphoid tissue. After a few hours,
they pass out of the blood back into the tissues by diapedesis.
Then, still later, they re-enter the lymph and
return to the blood again and again; thus, there is continual
circulation of lymphocytes through the body.
The lymphocytes have life spans of weeks or months;
this life span depends on the body’s need for these
cells.
The platelets in the blood are replaced about once
every 10 days; in other words, about 30,000 platelets
are formed each day for each microliter of blood.
Neutrophils and Macrophages
Defend Against Infections
It is mainly the neutrophils and tissue macrophages
that attack and destroy invading bacteria, viruses, and
other injurious agents. The neutrophils are mature
cells that can attack and destroy bacteria even in the
circulating blood. Conversely, the tissue macrophages
begin life as blood monocytes, which are immature
cells while still in the blood and have little ability to
fight infectious agents at that time. However, once they
enter the tissues, they begin to swell—sometimes
increasing their diameters as much as fivefold—to as
great as 60 to 80 micrometers, a size that can barely be
seen with the naked eye. These cells are now called
macrophages, and they are extremely capable of combating
intratissue disease agents.