Eosinophils: Overview, Quantity, Morphology

Overview

Origin

Eosinophils are produced in the bone marrow in a process similar to neutrophil production.

Eosinophils are recognizable at the myelocytes stage with the appearance of eosinophil specific granules.

IL-5 produced by sensitized T lymphocytes is the major cytokine that stimulates eosinophil production.

Marrow generation time and storage of eosinophils are similar to neutrophils.

Function

Eosinophils participate as a major component of systemic hypersensitivity reactions.

> When parasite antigens or allergens bind to specific IgE on mast cells, the mast cells degranulate and release histamine which attracts eosinophils.

Eosinophils play a major role in killing flukes and nematodes that have IgG or complement bound to their surface.

Eosinophils have limited phagocytic and bactericidal activity and may play a role in destroying neoplastic cells.

Quantity

Eosinophil numbers in circulation reflect a balance of marrow production and demand or consumption in tissue.

> A variety of diseases are characterized by a marked eosinophilic inflammatory cell response in tissue. These diseases may or may not be accompanied by an eosinophilia in blood.

There is considerable geographic variation in reference ranges for eosinophils.

> Ranges are higher in the south and east coastal regions of North America than for the northern and central plains areas.

> It is important to use species reference ranges generated from animals resident in the region.

Eosinopenia

Eosinopenia is defined as a reduction in circulating eosinophils.

> In most laboratories, the lower limit for absolute number of eosinophils is zero (0) or a very small number. Therefore, eosinopenia is best detected in sequential CBCs.

Endogenous secretion or administration of glucocorticoids results in eosinopenia by reducing marrow release and increasing eosinophil sequestration and apoptosis in tissue.

Because eosinophils are reduced with increased glucocorticoids levels, causes of eosinophilia should be considered in a stressed animal with a normal eosinophil count.

Eosinophilia

Eosinophilia is defined as an increase in absolute eosinophil count.

Causes of eosinophilia include:

• Allergy - IgE-mediated hypersensitivity reactions are likely causes especially when the animal experiences re-exposure to the inciting allergen.
• Parasites - Flukes, nematodes, or ectoparasites that have significant tissue migration or tissue contact phase.
• Granulomatous inflammation - Chronic granulomatous disease caused by fungi or foreign bodies.
• Neoplasms - Mast cell tumors and less frequently lymphomas can evoke an eosinophilia. A neoplasm that is heavily infiltrated with eosinophils may have a more favorable prognosis that one that is free of eosinophils.
• Hypereosinophilic syndrome - Difficult to differentiate from eosinophilic leukemia. Usually considered after other causes are eliminated.
• Eosinophilic leukemia - Extremely rare myeloproliferative neoplasm in cats. Eosinophilia is present with immature forms in blood. Eosinophil infiltrates are also seen in liver, spleen, and lymph nodes.

Allergic, parasitic, or inflammatory diseases that occur in the skin, gastrointestinal tract, lung, or female genital tract are most likely to produce eosinophilia.

Morphology

There are marked species differences in eosinophil morphology.

Canine Eosinophils

> Size - 12 - 20 μ in diameter or similar or slightly larger than a neutrophil.

> Nucleus - lobulated or partially segmented with dense, dark purple chromatin.

> Cytoplasm - orange-red granules that are round and vary in size and number (Fig. 6-1a and Fig. 6-1b. On occasion, canine eosinophil may contain one large solitary round granule that is similar in size and color to a RBC (Fig. 6-1c).

Figure 6-1a. Canine eosinophils. Granules in canine eosinophils are round and can vary in size and number. Granules are numerous, round, and small.

Figure 6-1b. Canine eosinophils. Granules in canine eosinophils are round and can vary in size and number. A few granules are present and size variation is more pronounced.

Figure 6-1c. Canine eosinophils. Granules in canine eosinophils are round and can vary in size and number. This eosinophil contains a few large round granules. In this cell, the granules appear similar in size and color to the surrounding RBCs (100x).

Feline Eosinophils (Fig. 6-2a, Fig. 6-2b, and Fig. 6-3)

> Size - 12-20 μ in diameter or, similar or slightly larger than a neutrophil.

> Nucleus - lobulated or segmented with dark purple chromatin.

> Cytoplasm - numerous, light pink rod-shaped granules.

Figure 6-2a. Feline eosinophils. Eosinophil (top) has granules that are pale pink and rod-shaped.

Figure 6-2b. Feline eosinophils. The cell membrane has ruptured in smear preparation and the rod-shaped granules are clearly visible (100x).

Figure 6-3. Feline eosinophilic leukemia. Two neutrophils (arrows) are adjacent to several eosinophils at various stages of development that include segmented, band, metamyelocyte, and myelocyte forms (100x).