05: WBC

Types of WBCs

• Neutrophils

• Eosinophils

• Basophils

• Monocytes

• Lymphocytes

WBC that doesn’t proliferate in bone marrow

Lymphocytes (→ lymphoid tissue!)

Most prevalent WBC in most animal species

Neutrophils

Most prevalent WBC in ruminants

Lymphocytes

WBCs classified as granulocytes

• Neutrophils

• Eosinophils

• Basophils

How do neutrophils stain

Basophilic nucleus with neutral granules that stain really poorly and are faintly pink/purple

How do eosinophils stain

Basophilic nucleus with bright red/orange granules

How do basophils stain

Basophilic with bright blue/purple granules

Which granulocyte is also called a polymorphonuclear leukocyte

Neutrophil

WBCs classifies as mononuclear

• Monocytes

• Lymphocytes

Monocyte function

Becomes macrophages in tissues, indicator of chronic inflammation

Lymphocyte function

Immune responses (B and T cell proliferation)

Where do lymphocytes live

• LNs

• Spleen

• Other lymphoid tissue

Neutrophil function

Acute inflammation

Eosinophil function

Worms, wheezes, and weird diseases

• Hypersensitivity reactions

Basophil function

Shows up with eosinophils

What is an analyzer counting when it gives the WBC count

Total amount of nucleated cells

Why is the WBC not always accurate

In regenerative anemias, there are nRBCs that get counted as WBCs

We ignore the WBC. What do we look at instead

Cell differentials

The cell differential is reported in two ways, which do we use clinically

Absolute number (discard the %)

I repeat: WHICH PART OF THE CELLULAR DIFFERENTIAL DO YOU INTERPRET

Absolute count (don’t disappoint Meinkoth)

Test done to confirm the WBC numbers from the analyzer

Blood smear :))

Additional benefit of doing a blood smear for WBCs

Allows you to look at morphology

Common changes in WBC morphology

• Left shift (younger)

• Toxic change (manufacturing errors)

• Larger

What part of the blood smear do you look at to estimate WBC number

Monolayer

Why do analyzers get differentials wrong in sick animals

The machine is really bad when it comes to abnormal morphologies, and things get categorized incorrectly

Main use of looking at a dot plot/histogram from a CBC report

If the segments all run together, there is an error in the analyzer differential count

Term for immature neutrophils

Bands

Characteristics of band morphology

No segments in their nuclei

When are bands present

When there is lots of inflammation and the bone marrow is kicking out neutrophils before they mature all the way

How to differentiate bands and monocytes on a blood smear (since they both look mononuclear)

Monocytes are larger and have darker blue cytoplasm, often with distinct vacuoles

Characteristics of neutrophils/bands with toxic change

Darker blue cytoplasm with foamy and indistinct vacuoles, ± ring nuclei

When do we see toxic changes

With bands; when the bone marrow is cranking out cells so fast that there are production errors, and leaking lysosomal enzymes cause damage in the cytoplasm

How are toxic changes rated

1-4

Where do neutrophils get produced

Bone marrow

How are neutrophils cleared

Tissue use

Where do neutrophils carry out their function

In tissues, not blood

T/F: low neutrophil count means low production

False, it could also mean really high demand in the tissues

How quickly can neutrophil counts change

VERY

What is meant by a “left shift” of neutrophils

The cell population is less mature because the marrow cannot meet the demand, and there are less segs and more bands, maybe even metamyelocytes

Three neutrophil populations in the bone marrow

• Proliferating: actively dividing

• Maturation: no division, just maturing

• Storage pool

Neutrophil storage pool function

Provides 2-3 days of neutrophils, an “emergency fund”

Two neutrophil populations in the blood

• Circulating pool

• Marginated pool

Why are some blood neutrophils marginated

They are temporarily stuck to the vessel wall, sensing the ECF for inflammatory mediators that would signal for the neutrophil to extravasate

T/F: the proliferation pool and the maturation pool are roughly the same size

False; immature cells produce exponentially more mature cells

How long does it take for the bone marrow to produce and release a mature neutrophil

4-6 days

• 2-3 days to reach the maturation pool

• 2-3 days to mature for relase

In most animals, how does the size of the circulating pool compare to that of the marginated pool

Roughly equal

How does the size of the circulating pool compare to the marginated pool in cats

The marginated pool is 2x the size of the circulating pool

When we get a WBC differential of neutrophils, what is that number measuring

Neutrophil count in the circulating pool

Mechanisms of neutrophilia

• Epinephrine/physiologic response

• Glucocorticoid/stress response

• Inflammatory response

Which type of leukogram is most important to discern from the prior list

Inflammatory leukogram

How does epinephrine cause neutrophilia

Instantaneously and temporarily causes the marginated neutrophils to move into circulation

How high can an epinephrine-induced neutrophilia get

2x reference interval, up to 3x in cats

What other WBC change will be seen with an epinephrine leukogram

Mature lymphocytosis up to 2x reference interval

How do glucocorticoids cause neutrophilia

• Redistributes marginated pool to circulation

• Some mobilization of the storage pool

• Stops neutrophils from leaving the blood

How high can a glucocorticoid-induced neutrophilia get

2x reference interval, 3x in cats

What other WBC change will be seen with a glucocorticoid leukogram

Lymphopenia

Why do glucocorticoids cause lymphopenia

They cause redistribution of lymphocytes to lymphoid tissue and also kill lymphoblasts

How does inflammation cause neutrophilia

• Mobilization of storage pool

• Proliferation in bone marrow

Why does inflammation cause bone marrow proliferation

Inflammatory cytokines cause an increase in WBC production

What type of change may be seen with a neutrophilia that immediately tells you its inflammation

Left shift

How high can an inflammatory neutorophilia get

Any magnitude

What species has an almost non-existent bone marrow response

Cattle

Criteria for a left shift

• Bands above reference interval, AND/OR

• Bands >10% of neutrophil differential

Types of left shifts

Regenerative and degenerative

Regenerative left shift

Neutrophilia where there are more mature/segs than bands

Degenerative left shift

• Neutrophilia with more bands than segs, OR

• Neutropenia with a left shift

Pelger-Huet Anomaly

Inherited condition where the neutrophils never segment

What will you see on a blood smear from a patient with Pelger-Huet

Every neutrophil will have a band-like nucleus, but they will have a perfectly clear cytoplasm with no vacuoles

What is the rare 4th mechanism of neutrophilia

Neoplastic neutrophilia

Causes of neoplastic neutrophilia

• Neutrophilic/granulocytic leukemia

• Paraneoplastic neutrophilia

Categories of granulocytic leukemia

• Acute: lots of blast cells

• Chronic: lots of mature cells

When to consider a neoplastic neutrophilia instead the three common mechanisms

• MARKED neutrophilia

• No obvious explanation (doesn’t fit the three common mechs)

List the two mechanisms for neutropenia

• Lack of production

• Increased tissue consumption

Signs associated with neutropenia from increased consumption

• Very sick patient (always look at the patient in front of you!)

• Left shift

• Toxic change

Signs associated with neutropenia from low marrow production

Pancytopenia → if the bone marrow can’t make neutrophils, it can’t make anything

• No left shift: the cells aren’t coming out early, they just aren’t coming out at all

Which WBC differential will not drop if you have low marrow production

Lymphocytes

Where do lymphocytes proliferate

Lymphoid tissue

Causes of lymphopenia

Stress leukogram, so common that it is usually dismissed

If a very stressed/sick animal has no stress leukogram (normal lymphocyte count), what alarm bell should be going off in your head

Hypoadrenocorticism (Addison’s disease)

• Steroids kill lymphoblasts, no steroids → normal lymphocyte count

Three lymphocytosis mechanisms

• Epinephrine leukogram

• Chronic disease

• Neoplasia

How does epinephrine cause lymphocytosis

Demargination, causing a mild increase

How does chronic disease cause lymphocytosis

Immune stimulation, usually mild

Types of cancers that cause lymphocytosis

Lymphoid leukemia (bone marrow) and leukemic lymphoma (lymphoid tissue)

How high with a neoplastic lymphocytosis get

HIGH (>>>20k)

Reasons to do a blood smear for a patient with lymphocytosis

Can help you diagnose abnormalities in the lymphocyte population

Characteristics of abnormal lymphocytes

• Larger than neutrophils

• Immature: less condensed chromatin → lighter purple nucleus

• LARGE nucleus

Causes of monocytosis

Stress leukogram or acute/chronic inflammation

Clinical significance of monocytopenia

It’s not

Causes of eosinophilia

• Parasites

• Hypersensitivity reaction

• Neoplasia

If you have an eosinopenia in a patient that is very sick, what alarm bells should be going off

Hypoadrenocorticism (Addison’s, again)