leukocyte responses in disease

5 major types of leukocytes in circulation

leukocytes

use blood as transport system

production/ release stimulated by inflammatory cytokines from injured/ infected areas

granulocytes

have specific cytoplasmic granules and lobulated nuclei: neutrophil, eosinophil, basophils

agranulocytes

lymphocytes and monocytes

interpretation of leukocyte responses

Most leukocyte response patterns are non-specific → reflect basic underlying process (e.g. inflammation, stress, etc.)

Haematology alone does not give you an aetiological diagnosis, except haematopoietic neoplasia.

Total white blood cell count is of little use.

A WBC differential count is absolutely required!

Haematology analysers may not produce an accurate WBC diff count.

There is no substitute for blood smear examination by a clinical pathologist

leukocytosis

increased WBC count

leukopenia

decreased WBC count

leukocyte kinetics and pools

BM: precursors for all leukocytes

Lymphoid tissues: precursors for lymphocytes

Blood: leukocytes in transit from sites of production to sites of function or destruction

Tissues

• Granulocytes perform their roles and die.

• Lymphocytes may return to blood via lymphatic vessels or die.

• Monocytes transform into macrophages or dendritic cells that perform their functions and die.

neutrophils

Segmented neutrophil

• ≃ 12 μm in diameter

• Lobulated nucleus (3-5 lobules)

• Clear to pale pink cytoplasm

Maturation time in BM (from blast to segmented neutrophil) is 5-7 days → shortened in inflammation

Many neutrophils are stored in the BM.

Blood T1/2 is 5-10 h.

✔MNP:CNP≃1 in the dog

✔MNP:CNP≃3 in the cat

Migration to tissues

Neutrophils perform their functions and die or they are lost across mucosal surfaces

Live only for a few days in tissues

left shift

The hallmark of acute/active inflammation

Great demand → SNP diminishes → Release of immature NEUTs from MatNP

Band neutrophil

• U or S shaped, non-segmented nucleus

• Immature, one stage before segmented neutrophil

Metamyelocytes, myelocytes, promyelocytes, myeloblasts

Regenerative

• Segmented NEUTs > immature NEUTs

Degenerative

• Immature NEUTs > segmented NEUTs

• Poor prognostic indicator

Pelger-Huët anomaly

• Hyposegmented granulocytes

• Hereditary disorder

toxic changes

Foamy cytoplasm

Diffuse cytoplasmic basophilia

Döhle bodies

Asynchronous nuclear maturation

Giant neutrophils

Toxic granules

Maturation defects occurring during accelerated neutropoiesis

neutrophilia - aetiology

inflammation - inflammatory neutrophilia

excess of catecholamines

• physiological neutrophilia

excess of glucocorticoids

• stress neutrophilia

rare causes

• chronic myeloid leukaemia

• paraneoplastic syndrome

Inflammatory neutrophilia

Infection, immune-mediated disease, neoplasia, necrosis

Toxic changes may be seen.

Left shift → acute/active inflammation

Inflammation of the CNS, superficial skin and lower urinary tract may not be associated with neutrophilia

stress neutrophilia

Excess of endogenous or exogenous glucocorticoids

Shift of NEUTs from MNP to CNP

Increased release of NEUTs from BM

Magnitude: x2-4, less pronounced in cat

Lymphopenia is common, monocytosis and eosinopenia can be seen, mild left shift uncommon

Physiologic neutrophilia

Excess of catecholamines

• Fear, excitement, exercise

Shift of NEUTs from MNP to CNP

Transient (back to normal within an hour if stimulus disappears)

More common in cats

Magnitude: up to x2 (dog) or x4 (cat)

Lymphocytosis and infrequently monocytosis can be seen

neutropenia - aetiology

increased use

decreased/ ineffective production

other

• endotoxaemia

• immune-mediated neutropenia

• haemophagocytic syndrome

• cyclic haematopoiesis of grey collies and FeLV infected cats

Neutropenia – increased use

Overwhelming acute inflammation

Migration of NEUTs to inflamed tissue exceeds the release from BM.

Left shift is typically present.

Toxic changes may be present.

Mostly in bacterial infections of lungs, uterus, GI tract and body cavities

neutropenia - inadequate production

Bone marrow disorder → NEUTs first to be affected.

Persistent neutropenia, typically without left shift (unless secondary

infection)

Infections

• Parvovirus, FeLV, Ehrlichia canis

Drug toxicity

• Oestrogen, chemotherapy, idiosyncratic reactions

Myelofibrosis, myelophthisis

lymphocytes

Small, mature lymphocytes

• ≃ 7-8 μm in diameter

• Round nucleus, clumped chromatin, no visible nucleolus

• Scant amount of lightly basophilic cytoplasm

Intermediate and large lymphocytes also exist.

Second most common leukocyte

Produced in BM or primary lymphoid tissues

T cells mature in thymus.

B and NK cells mature in BM and lymphoid tissues.

Blood: CLP and MLP

They can re-enter lymphoid tissues

Migration to other tissues to perform their actions → they die or can return to blood

T1/2 varies from hours to years

Lymphocytosis – aetiology

chronic inflammation/ antigenic stimulation

excess of catecholamines - physiologic lymphocytosis

lymphoproliferative disease

hypoadrenocorticism - reverse stress leukogram

young animals

Chronic inflammation/antigenic stimulation

Usually mild to occasionally moderate (rarely up to 30 x 109/L in dogs)

Reactive lymphocytes may be present.

Neutrophilia common, monocytosis and eosinophilia may be seen

Physiologic lymphocytosis

See physiologic neutrophilia

Magnitude: up to x2

Granular lymphocytes may be seen in increased numbers

Lymphoproliferative disease

Neoplastic disease

Chronic lymphocytic leukaemia

Leukaemic phase of lymphoma

Mild to extreme increase in LYMPH count

lymphopenia - aetiology

excess of glucocorticoids - stress lymphopenia

• most frequent cause

acute inflammation

• stress

• other mechanisms

depletion

• loss of lymphocyte-rich fluid e.g enteric disease, repeated drainage of chylothorax

hypoplasia/ destruction/ distruption of lymphoid tissue

• multicentric lymphoma, immunosuppressive drugs

monocytes

Same size to NEUTs or slightly larger

Variably shaped nucleus

Lightly basophilic or amphophilic cytoplasm can contain vacuoles

Low numbers in health

Kinetics similar to NEUTs

Produced in BM

Blood: T1/2 of 0.5-3 days (CMP – MMP)

Migrate to tissues

Transformed to macrophages or dendritic cells → might live for 10 days to >1 year

monocytosis - aetiology

inflammation

• acute or chronic

excess of glucocorticoids

secondary to neutropenia

• NEUTs and MONOs share common bipotential stem cell

chronic myeloid leukaemia

• very rare

Eosinophils

Same size to NEUTs or slightly larger

Lobulated nucleus similar to NEUTs

Orange/red cytoplasmic granules

Low numbers in health

Kinetics similar to NEUTs

Produced in BM → blood (T1/2 of 8-18 h) → tissues (may remain for weeks to months there)

eosinophilia - aetiology

hypersensitivity reactions

• flea-bite dermatitis, feline asthma, food allergy

parasitism

• ecto and endo parasites

inflammation

• idiopathic conditions - eosinophilic enteritis, eosinophilic bronchopneumonia, eosinophilic panostetis

• non-specific finding in inflammation of mast cell-rich tissues

neoplasia

• chronic eosinophilic leukaemia or paraneoplastic e.g MCT, lymphoma

hypereosinophilic syndrome = persistent, marked eosinophilia

hypoadrenocorticism = reverse stress leukogram

basophils

Same size to NEUTs

Lobulated nucleus

Purple cytoplasmic granules

Rare in health

Kinetics similar to NEUTs

Produced in BM → blood (T1/2 of 2-3 days) → tissues (may remain for a few days)

basophils

mast cells

Quite large compared to other leukocytes

Generally round nucleus

Purple cytoplasmic granules

Not seen in health

Produced in BM → blood as undifferentiated cells → tissues (long-lived cells)

mast cells

basophilia

Typically accompanies eosinophilia

Hypersensitivity reactions

Parasitism

Neoplasia

mastocytosis

Even one mast cell in blood smear or BC smear

Many → mast cell tumour

Rare/few → have also been reported in several acute inflammatory diseases in dogs; still strongly suggestive of mast cell tumour in cats

Monocytopenia

Healthy animals can have only rare monocytes; not a diagnostic problem

eosinopenia

Stress leukogram; however, healthy animals can have only rare eosinophils

basopenia

typically very rare in any case

Haematopoietic neoplasia

Clonal proliferation of lymphocytes (lymphoid origin) or other leukocytes and their precursors (myeloid in origin)

Typically associated with marked leukocytosis, but reduced numbers of other leukocytes

Acute leukaemia

• Poorly differentiated, overtly abnormal cells

Chronic leukaemia

• Well-differentiated cells

Leukaemic phase of lymphoma

• Starting from lymphoid tissues and infiltrating the blood