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White Blood Cells
White blood cells, or leukocytes, are immune cells that are present
in the blood. The detailed mechanisms of immune function are covered
in the immunity unit, but we will discuss the classes here. There are five
common types of leukocytes (and some of those types have subgroups).
One way of categorizing these five is by whether or not they contain
granules in their cytoplasm when the cells are stained. If they do, they
are a type of granulocyte. If not, they fall into the category of
agranulocytes. Another way of distinguishing leukocytes from one
another is by the morphology (shape and patten) of their nuclei.
Granulocytes have multilobulated nuclei and agranulocytes have a
spherical (nonlobulated) nucleus. The lifespan of a granulocyte ranges
from about 12 hours to 4 days, but the agranulocyte survives for
approximately 100 to 300 days.
Leukocytes mainly function to protect the body from microbial invasion
and toxins. WBCs readily travel through the bloodstream. However,
certain leukocytes have the ability to move into the interstitial spaces of
the body’s tissues in order to attack foreign agents to protect the body
from infection. They leave the capillalry by squeezing through the pores
in the capillary walls via a process called diapedesis. This is due to the
ability of these leukocytes to rearrange cytoskeletal matrix in a manner
that changes their shape.
Granulocytes
Granulocytes contain many granules within the cytoplasm, and they
have a multilobular, irregularly shaped nucleus. There are three types of
granulocytes: neutrophils, eosinophils, and basophils. Specific stains
used to reveal the granules in their cytoplasm identify each granulocyte.
Granulocytes are responders of the nonspecific immune system, which
means that the cells respond to injury and build up an inflammatory
response against microbes or toxins regardless of whether the body has
been previously invaded by the foreign agent. That is to say,
granulocytes do not possess memory when mounting an immune
response.
Nonspecific Immune Response Leukocytes
Neutroph
il
Eosinoph
il
Basophil
Monocyt
e
Neutrophils
Neutrophils are the most abundant type of granulocyte (50 to 70
percent of circulating WBCs). The S- or C-shaped nucleus of these cells
has three to six lobules that are connected via strands of chromatin. The
multi-lobular shape of the nucleus often classifies neutrophils
as polymorphonuclear leukocytes (PMNs).
During a bacterial infection, neutrophils are the first responders. The
PMNs arrive at the site of infection through a process known
as chemotaxis in which chemicals released by damaged cells draw
neutrophils. The neutrophils attack bacteria by engulfing them via the
process of phagocytosis. Once the bacteria are contained within the
neutrophils, the granules release antimicrobial compounds
called defensins andlysozymes to destroy the microbes. Pus is the
culmination of destroyed neutrophils, bacteria, and dead tissue.
Eosinophils
Eosinophils only possess a bilobed nucleus linked by a thin strand of
chromatin. Unlike neutrophils, eosinophils stain red with an eosin dye;
the dye is acidic and is picked up by the cytoplasmic granules that
contain digestive enzymes. Eosinophils also engulf foreign molecules
attacking the body, particularly in parasitic infection and allergic
reactions. They also play a role in modulating the activity of basophils.
Basophils
The nucleus of basophils can contain two to five U- or S-shaped
lobules. These leukocytes pick up basic dyes that cause their granules to
have a blue-purple appearance. The granules of basophils contain
histamine, serotonin, and heparin. The cells release histamine during an
inflammatory response to tissue damage and microbial invasion.
Basophils are similar in appearance and function with mast cells, which
are strictly localized in connective tissue.
Agranulocytes
Unlike granulocytes, agranulocytes do not have granules within their
cytoplasm and lack a lobulated nucleus. Under a microscope
agranulocytes are observed to have a nucleus that composes the bulk of
its cellular volume. Agranulocytes include lymphocytes, which are
responsible for the specific immune response, meaning that they have
memory and build up a vigorous response against toxins or
microorganisms that the body has encountered before. The other
agranulocyte is the monocyte, which is an immature form of the nonspecific macrophage.
Lymphocytes
Lymphocytes can be categorized by their size (small, medium, and
large). These cells have a round nucleus that virtually fills up the cell
leaving very little blue-staining cytoplasm surrounding it (when stained
in a standard manner). Lymphocytes are the only WBCs that reenter the
bloodstream after moving into tissue to attack foreign agents.
There are two types of lymphocytes: T lymphocytes (T cells) and B
lymphocytes (B cells).
They both originate in the bone marrow, but T cells grow and mature
within the thymus gland while B cells typically mature in bone marrow
then move to lymph nodes and similar tissue. T cells attack abnormal
cells in the body, such as virally infected cells, tumor cells, and donor
transplant cells. B cells sense and attack invading antigens (foreign
substance in the body) such as toxins, viruses, or bacteria. In response
to antigens, B cells divide into plasma cells that make antibodies aimed
at neutralizing and destroying these antigens.
Monocytes
The nuclei of monocytes are also large, but have a kidney shape. The
cytoplasm of monocytes is greater in quantity compared to lymphocytes,
and it stains in a bluish-gray color. Monocytes have the capacity to leave
the bloodstream and enter tissue, where they mature into cells
called macrophages.
Macrophages
phagocytose
(phagocytize)
microbes and tissue debris.