White blood cells (leukocytes). Granulocytes
Granulocytes, the most numerous of the white cells,
are larger than red cells (approximately 12–15 micrometres).
They have a multilobed nucleus and contain large numbers
of cytoplasmic granules (i.e., granules in the cell
substance outside the nucleus). Granulocytes are important
mediators of the inflammatory response. There are three
types of granulocytes: neutrophils, eosinophils, and
basophils. Each type of granulocyte is identified by
the colour of the granules when the cells are stained
with a compound dye. The granules ofthe neutrophil are
pink, those of the eosinophil are red, and those of
the basophil are blue-black. About 50 to 80 percent
of the white cells are neutrophils, while the eosinophils
andbasophils together constitute no more than 3 percent.
The neutrophils are fairly uniform in size with a diameter
between 12 and 15 micrometres. The nucleus consists
of two to five lobes joined together by hairlike filaments.
Neutrophils move with amoeboid motion. They extend long
projections called pseudopods into which their granules
flow; this action is followed by contraction of filaments
based in the cytoplasm, which draws the nucleus and
rear of the cell forward. In this way neutrophils rapidly
advance along a surface. The bone marrow of a normal
adult produces about 100,000,000,000 neutrophils daily.
It takes about one week to form a mature neutrophil
from aprecursor cell in the marrow, yet once in the
blood the mature cells live only a few hours, or perhaps
a little longer after migrating to the tissues. To guard
against rapid depletion of the short-lived neutrophil
(for example, during infection), the bone marrow holds
a large number of them in reserve to be mobilized in
response to inflammation or infection. Within the body
the neutrophils migrate to areas of infection or tissue
injury. The force of attraction that determines the
direction in which neutrophils will move is known as
chemotaxis and is attributed to substances liberated
at sites of tissue damage. Of the 100,000,000,000 neutrophils
circulating outside the bone marrow, half are in the
tissues and half are in the blood vessels; of those
in the blood vessels, half are within the mainstream
of rapidly circulating blood and the other half move
slowly along the inner walls of the blood vessels (“marginal
pool”), ready to enter tissues on receiving a chemotactic
signal from them.
Neutrophils are actively phagocytic; they engulf bacteria
and other microorganisms and microscopic particles.
The granules of the neutrophil are microscopic packets
of potent enzymes capable of digesting many types of
cellular materials. When a bacterium is engulfed by
a neutrophil, it is encased in a vacuole lined by the
invaginated membrane. The granules discharge their contents
into the vacuole containing the organism. As this occurs,
the granules of the neutrophil are depleted (degranulation).
A metabolic process within the granules produces hydrogen
peroxide and a highly active form of oxygen (superoxide),
whichdestroy the ingested bacteria. Final digestion
of the invading organism is accomplished by enzymes.
Eosinophils, like other granulocytes, are produced
in the bone marrow until they are released into the
circulation. Although about the same size as neutrophils,
the eosinophil contains larger granules and the chromatin
is generally only concentrated in two nonsegmented lobes.Eosinophils
leave the circulation within hours of release from the
marrow and migrate into the tissues (usually those of
the skin, lung, and respiratory tract) through the lymphatic
channels. Like neutrophils, eosinophils respond to chemotactic
signals released at the site of cell destruction. They
are actively motile and phagocytic. Eosinophils are
involved in defenseagainst parasites, and they participate
in hypersensitivity and inflammatory reactions, primarily
by dampening their destructive effects.
Basophils are the least numerous of the granulocytes,
and their large granules almost completely obscure the
underlying, double-lobed nucleus. Within hours of their
release from the bone marrow, basophils migrate from
the circulation to the barrier tissues (e.g., the skin
and mucosa), where they synthesize and store histamine,
a natural modulator of the inflammatory response. When
aggravated, basophils release, along with histamine
and other substances, leukotrienes, which cause bronchoconstriction
during anaphylaxis (a hypersensitivity reaction). Basophils
incite immediate hypersensitivity reactions in association
with platelets, macrophages, and neutrophils
