Basophils: Overview, Quantity, Morphology

Overview

Origin

Basophils are produced in the bone marrow and share a common progenitor cell with tissue mast cells.

Basophils do not develop into mast cells, but the two cell types have similar functions.

Immature basophils can be recognized at the myelocytes stage by their characteristic secondary granules.

Maturation through the metamyelocyte, band, and segmented stages in marrow takes about 2.5 days.

Basophils circulate for a few hours in blood and migrate into tissues where they may reside for several weeks.

Function

Basophil granules contain histamine and heparin.

> Histamine release from basophils and mast cells plays a significant role in immediate hypersensitivity reactions such as occurs in urticaria, anaphylaxis, and acute allergy.

> Heparin inhibits coagulation, which has an important role in inflammation.

> Activated basophils synthesize several cytokines that initiate or modulate the inflammatory response.

Quantity

Basophils comprise a very low percentage of the circulating leukocyte population. In most dogs and cats, basophils will rarely be observed in a manual leukocyte differential count.

Basopenia

Since basophils are seen rarely in peripheral blood, it is difficult to assess basopenia. Endogenous or exogenous glucocorticoids cause a reduction in circulating basophils.

Basophilia

Causes of basophilia should be considered in a dog or cat with a persistent basophil count of 200 - 300/μl. Basophilia often occurs concurrently with eosinophilia.

Causes of basophilia include:

• Allergy and hypersensitivity reaction
• Parasites - Nematodes, flukes, or ectoparasites that have a significant tissue migration or tissue contact phase. Dirofilaria, tick infestation, or flea allergy are frequent causes.
• Hyperlipemia - Metabolic or endocrine disorders associated with lipemia may be accompanied by basophilia.
• Basophilic leukemia - Extremely rare myeloproliferative neoplasm

Morphology

There are marked species differences in basophil morphology.

Canine Basophil

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

> Nucleus - segmented lobulated nucleus (Fig. 7-1a)

> Cytoplasm - light purple or grey with a few discrete dark granules (Fig. 7-1b). Granules may be very sparse or absent in some cells.

Figure 7-1a. Canine basophils. Lobulated nucleus is present in a grey cytoplasm with a few basophilic granules.

Figure 7-1b. Canine basophils. The basophil (left) has darker cytoplasmic granules compared with the orange pink granules in the eosinophils (100x).

Feline Basophil

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

> Nucleus - segmented lobulated nucleus with light smooth chromatin very similar to the nucleus in a monocyte.

> Cytoplasm - numerous round, lavender granules (Fig. 7-2a and Fig. 7-2b).

Figure 7-2a. Feline basophil. Numerous round lavender granules surround a lobulated nucleus. Feline basophils can be easily confused with monocytes.

Figure 7-2b. Feline basophil. The cell membrane has ruptured in smear preparation and the lavender cytoplasmic granules are clearly visible (100x).

Mast Cells

> Size - similar to basophils in cat and dog.

> Nucleus - round to oval, central or eccentric location in cytoplasm.

> Cytoplasm - numerous small round, dark purple granules that may partially obscure the nucleus (Fig. 7-3 and Fig. 7-4).

Mast cells are rarely seen in peripheral blood.

Figure 7-3. Mast cell leukemia in a cat. Mast cells have a round nucleus with a moderate amount of cytoplasm. This mast cell contains only a few basophilic cytoplasmic granules (100x).

Figure 7-4. Fine needle aspirate of a lymph node. Metastasis of a cutaneous mast cell tumor to a regional lymph node. Numerous well-granulated mast cells are present with a few small lymphocytes (60x).