Download THE WWW OF WHITE BLOOD CELLS: CLINICAL PATHOLOGY

THE WWW OF WHITE BLOOD CELLS:
WONDERFUL, WICKED WORLD
Brandy Sprunger-Helewa, CVT, RVT, AAS, VTS (ECC)
CLINICAL PATHOLOGY/NURSING
Despite all the different types of white blood cells the body creates, they all are involved in the same system of the body:
the immune system. They are designed to fight pathogens, whether inhaled, injected, digested, or entered the body in some
other manner. They develop memory as well, so that the next time the same pathogen is introduced, it can be destroyed
even faster. All white blood cells originate from the same multipotent stems cells as all other cells in the body. From there,
they differentiate into myeloid or lymphoid cells within the bone marrow. Lymphocytes are the only white blood cell that
are created from these lymphoid cells. All other white blood cells come from the myeloid line, which is also where
erythrocytes and platelets branch off. From here, they become myeloblasts, or the precursors to the other types of white
blood cells: monocytes, neutrophils, basophils, and eosinophils.
Understanding the purpose of each cell line will help make sense of lab work results. What does it mean to have a high
neutrophil count? What if the lymphocytes are elevated? What if all the cells are low in number? These are important
things to understand so that we can knowledgably and skillfully care for our patients, whether hospitalized, undergoing a
procedure, or being treated as an outpatient.
There are two divisions of white blood cells: granulocytes and agranulocytes. Granulocytes include neutrophils, basophils,
and eosinophils. They are called granulocytes because of the appearance of granules within their cytoplasm. These
granules, which stain different colors in each cell type, are part of the lysosomes, which are used to help remove pathogens.
Granulocytes are also polymorphonuclear, meaning their nucleus is not simply round in shape; they can be lobed, squiggly
in appearance, or bean shaped. Agranulocytes, on the other hand, are mononuclear, meaning their nucleus is round in
shape, without lobes, indentations, or other curves. These include lymphocytes and monocytes. In mammals, neutrophils
are the predominant white blood cell, with lymphocytes second in total numbers. This is not true of birds or reptiles, where
the heterophils is the predominant white blood cell.
The term leukocytosis is defined by an increase in the total white blood cell count. Leukopenia, on the other hand, is
defined by a decrease in the total white blood cell count. Each cell type can be elevated or decreased based on whatever
disease process the patient is experiencing. It is possible to have a leukocytosis when only one type of white blood cell,
usually neutrophils, are elevated. It is also possible to have normal total white blood cell count despite one type of white
blood cell being significantly decreased in numbers. When all white blood cells, red blood cells, and platelets are
decreased, it is called pancytopenia. This is usually due to a malfunction of the bone marrow, either due to chemotherapy,
neoplasia, or bone marrow disease. If only the white cells are decreased, but red blood cells and platelets are normal, it is
called panleukopenia.
Neutrophils
In mammals, neutrophils comprise about 70% of total white blood cell counts. Their life span is approximately several
hours to five days; those neutrophils that move into tissues live longer than those circulating in blood. Their nucleus is
multilobed, and the granules of the cytoplasm stain a pale pink/purple color. At 10–12µm, they are medium in size
compared to other white blood cells. They tend to fight off fungi and bacteria, more than viruses, and they do this by
phagocytosis. They are the first white blood cell to respond to an insult, and can leave the blood stream and move into
tissues when needed there. Infected tissues release chemical signals that neutrophils read to know where they are needed.
In some instances where large numbers of neutrophils are required (pneumonia, infected wound, sepsis), the bone marrow
will send out immature neutrophils. These are known as bands, or band neutrophils. They are called this because of the
band-like or horseshoe shape of the nucleus. A complete blood count (CBC) will count these band neutrophils separately
from other neutrophils. When the band neutrophil number is elevated, it is known as a left shift. Hypersegmented
neutrophils have a nucleus with many turns, lobes, and folds; these are typically found in older, degenerating neutrophils.
The term “stress leukogram” can also be used when the overall number of neutrophils (and monocytes) is elevated; this is
usually due to an increase in endogenous or exogenous steroids, and can be a common finding in patients on steroid
therapy, or with hyperadrenocorticism. An increase in neutrophils is also known as neutrophilia. This is a common finding
in patients with infections, anemia, neoplasia, sepsis, or have had a sympathetic nervous system response (fight or flight;
most common in cats). A decrease in neutrophils is known as neutropenia, and can be seen in some neoplasias, bone
marrow disorders, starvation, medication use, and chronic inflammation or illness.
Because they cannot replace lysosomes that have been used to destroy pathogens, neutrophils break down and die off after
they have performed this function. This is what forms the appearance of pus. Once the neutrophil has phagocytized the
pathogen, macrophages engulf and digest them after receiving a chemical signal from the neutrophil that the pathogen has
been ingested. The breakdown of the pathogen by the neutrophil produces hydrogen peroxide, before it is quickly broken
down into other products.
Lymphocytes
In mammals, lymphocytes comprise about 20% of total white blood cell counts. Their life span is weeks to years; those
lymphocytes involved in memory, B cells, live longer than T cells or natural killer (NK) cells. Their nucleus is round,
deeply staining, and takes up the majority of the cell body. The cytoplasm stains a light pink/purple color. They are
involved in both cellular (T cell) immunity and humoral (B cell) immunity. At 7–8µm, they are small in size compared to
other white blood cells, though some lymphocytes can be 12–15µm. They are typically found in lymph and lymph tissues
of the lymph nodes, thymus, and spleen. Lymphocytes have implications in all forms of hypersensitivity reactions, but
particularly in delayed hypersensitivities like serum sickness.
A decrease in lymphocytes is known as lymphopenia. Common causes are radiation, chemotherapy, sepsis, burns,
pancreatitis, and protein losing enteropathy. An increase in lymphocytes is known as lymphocytosis. This is a common
finding in lymphoma, leukemia, autoimmune disease, or infections. Patients with lymphoma often present with very
enlarged lymph nodes.
T Cell Lymphocytes
T cells, or helper and cytotoxic cells, are involved in cellular immunity. They mature within the thymus. When a pathogen
is spotted, T helper cells send chemical messages to other cells in the immune system, which in turn results in an immune
response. T cytotoxic cells release toxic granules that kill the pathogen, or the pathogen infected cell. T helper cells are also
the fastest lymphocytes to respond when an antigen is presented again later, and is remembered by other lymphocytes. T
cytotoxic cells tend to destroy cancer and viral infected cells, though they can destroy other cells as well. T cells do not
require antibodies to be activated. There are also T suppressor cells, which work to return the immune system back to
normal after an insult, and prevent autoimmunity.
B Cell Lymphocytes
B cells are involved in humoral immunity. They mature within the bone marrow, though some become plasma (memory B)
cells when they move into the lymph nodes. When a pathogen is spotted, plasma cells develop antibodies, or
immunoglobulins, against them. These antibodies attach to the antigen, neutralizing and marking them for destruction by
other cells of the immune system. These memory B cells will forever remember every pathogen they have ever
encountered. This speeds up the immune response the next time the pathogen is spotted.
Natural Killer Cells
Natural killer cells are part of the innate immune system, and are the larger of the lymphocytes. This is immunity that is
present from birth, and does not involve the production of antibodies. They mature in the bone marrow, lymph nodes,
spleen, tonsils, and thymus. They destroy all cells in the same manner, and do not have memory of previous antigens. They
tend to destroy cancer or infected cells. They also release chemical messages when they target a pathogen, to instigate an
immune response from other cells.
Monocytes
In mammals, monocytes comprise about 5% of total white blood cell counts. Their life span is a few hours to years; those
that live in tissues, also called macrophages, live longer than those that circulate through the blood. Unlike neutrophils,
monocytes can replace their lysosomes, so are thought to live longer because of this ability. Their nucleus is usually kidney
shaped, or folded, but still round and stains a purple color; it may take up about half of the cell body. The cytoplasm stains
a pale purple color. At 12–15m, they are some of the largest cells compared to other white blood cells, with macrophages
being on the higher end of that scale. They are a large player in innate and cellular immunity, as one of their jobs is to
present pathogens to T cell lymphocytes for destruction. This also releases chemical messages that instigate a systemic
immune response. They are typically found in the bloodstream, spleen, lungs, and other tissues throughout the body.
Monocytosis is the term used when monocyte counts are elevated. This can be seen in cases of necrosis,
hyperadrenocorticism, stress (as part of the stress leukogram), immune diseases, and chronic inflammation. Monocytopenia
is the term used when monocyte counts are low; this is extremely rare, though can be a finding in some bone marrow or
neoplastic diseases, or with the use of chemotherapy.
Macrophages
When monocytes are needed at a site of infection, they quickly move from the blood stream and into the target tissue; this
can take less than 12 hours. They can phagocytize both pathogens directly, or infected cells. They also direct T cell
lymphocytes to destroy pathogens and infected cells as well. Once the pathogen has been phagocytized, by either
macrophages or monocytes, pieces of the pathogen are presented to B cell lymphocytes for the development of antibodies.
Mast Cells
Mast cells are permanent tissue residence made up of former monocytes. Their appearance is similar to that of a basophil,
though they are only found in tissues, not in blood where basophils are only found. The most common areas mast cells are
found are in the mucosa of the eyes, mouth, and nose. They play an important role in allergic reactions, anaphylaxis,
asthma, and rhinitis as they release their stores of histamine and heparin when attached to two IgE molecules
(immunoglobulin E). They can also release these chemicals when physically injured by alcohol, vigorous exercise, and
certain medications (opioids, antibiotics). Mast cell tumors to have the ability to proliferate both systemically and
cutaneously and form metastatic masses.
Kupffer Cells
Kupffer cells are former monocytes that set up permanent residence in liver cells, creating the reticuloendothelial system
(RES) or mononuclear phagocytic system (MPS). The RES is an important part of both cellular and humoral immunity,
and is also partly responsible for breaking down old erythrocytes and leukocytes. Kupffer cells also store the recycled iron
to be used during the creation of new red blood cells.
Histiocytes
Histiocytes are former monocytes that are also part of the RES, and look similar in appearance to eosinophils. They are
found in tissues throughout the body, and are sometimes called dendritic cells (due to their star shaped appearance, not
because of any relation to dendrites). They contain lysosomes, and phagocytize pathogens, while also presenting portions
of them to lymphocytes for antigen recognition. Many histiocytes move on to become Langerhans cells of the dermis and
lymph nodes. Histiocytosis, or a proliferation of histiocytes, can develop into small masses commonly seen on the limbs
and head of canines. They are usually benign and self-limiting.
Eosinophils
In mammals, eosinophils comprise about 2% of total white blood cell counts. Their life span is hours to days. Their nucleus
is usually bilobed and stain a deep purple. The granules within the cytoplasm stains a reddish pink/orange color. They are
involved in allergic responses, particularly seasonal allergies, asthma, and urticaria, though they are also the primary white
blood cell responsible for parasite infection response. At 10–12µm, they are medium in size compared to other white blood
cells. They are typically found in the thymus, GI tract, lymph nodes and spleen. They are driven to the sites under direction
of the T cell lymphocytes. They, in turn, present pathogen pieces to B cell lymphocytes for antibody creation. Their red
staining granules release enzymes, including hydrogen peroxide, that help destroy pathogens.
High levels of eosinophils is known is as eosinophilia, and is a common finding in patients with heartworm disease,
intestinal parasites, and allergic conditions like asthma. Eosinopenia is a low eosinophil count, and, though rare, can be
seen in patients with bone marrow disease, infections, or chemotherapy use.
Basophils
In mammals, basophils comprise less than 1% of total white blood cell counts. Their life span is a few hours to days; those
that move into tissues live longer. Their nucleus is bi- or trilobed and deeply staining. The cytoplasm granules stain a light
to dark blue in color. At 12–15µm, they are some of the largest cells in comparison to other white blood cells. They are
recruited by tissues from the blood stream, where they are involved in allergic responses, as their granules contain
histamine and heparin. They also respond to parasitic infections, similar to how eosinophils respond. Basophils are also
thought to send chemical messages to lymphocytes for antibody production.
Basophilia is the term used for an increased number of basophils. This can be seen in patients with allergic or parasitic
conditions, leukemia, or lymphoma, though it is rare. Basopenia, also rare, is a low basophil count, and is hard to diagnose
due to the low normal numbers of basophils in healthy patients.
Heterophils
Heterophils are found in some small mammals, avian and reptilian species, and are the most numerous leukocyte; they
essentially replace neutrophils on the leukogram. Their shape is more oblong than round, with a bi- or trilobed, purple
staining nucleus. The granules within the cytoplasm stain a light pink to red color, more similar to the granules of
eosinophils. They are the functional equivalent of neutrophils; they phagocytize pathogens and release several enzymes
including hydrogen peroxide. Heterophil activity and ability differs between species. Like neutrophils, band heterophils
may appear when the bone marrow sends immature heterophils into circulation. Heterophilia and heteropenia are the terms
used for an elevated heterophil count, and decreased heterophil count, respectively.
Knowledge of where each cell type comes from and their function can help us evaluate a CBC more effectively. If we
know why we may have changes to certain cell types, then we can perform the necessary nursing care required to bring the
numbers back to normal. Idexxlearningcenter.com is a great place to get additional continuing education (CE) on not only
blood smear evaluation, but on each cell type in particular.
References available upon request.