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Heterophil (Copyright © RVC)

Heterophils are the most abundant granulocyte in most avian species and occur alongside lymphocytes, monocytes, eosinophils and basophils in avian blood. These cells are also found in some reptile and mammalian species.


Heterophils have a similar development to other granulocytes; this process is called granulopoiesis.


The cytoplasm of normal heterophils appears colourless and contains eosinophilic granules (dark orange to brown-red) with Romanowsky stains. The cytoplasmic granules are elongated (rod or spiculated shape) but they may appear oval to round in some species.

The granules frequently have a distinct central body that appears to be refractile. The granules may be affected by the staining process and appear atypical.

The nucleus of mature heterophils is lobed (usually two to three) with coarse, clumped chromatin that stains purple. The nucleus is often partially hidden by the cytoplasmic granules.


Heterophils are functionally equivalent to neutrophils. They actively participate in inflammatory lesions and are phagocytic.

The cytoplasmic granules of heterophils contain lysozyme and proteins needed for bactericidal activity, although chicken heterophils lack peroxydase activity.


Immature Heterophils

These have increased cytoplasmic basophilia, non-segmented nuclei and immature granules compared with normal, mature heterophils.

The immature heterocytes most frequently encountered in blood are myelocytes and metamyelocytes. Myelocytes are larger than mature heterophils and they have a blue cytoplasm as well as secondary, rod-shaped granules and a round to oval non-segmented nucleus. Metamyelocytes resemble myelocytes except that the nucleus is indented and the rod-shaped granules occupy more than half the cytoplasmic space.

Band heterophils resemble mature heterophils, except that the nucleus is not lobed. The nucleus is often hidden behind the cytoplasmic granules, and it can be difficult to identify a band heterophil. Special stains can help with this.

Toxic Heterophils

Heterophils exhibit toxic changes in response to severe systemic illness.

These changes are quantified as to the number of toxic cells and the severity of the toxicity.

Toxic heterophils have increased cytoplasmic basophilia, vacuolisation, abnormal granulation (degranulation, deeply basophilic granules, coalescing granules) and degeneration of the cell nucleus.

The degree of toxicity can be rated on a scale of 1+ to 4+:

1+ toxicity: heterophils exhibit increased cytoplasmic basophilia
2+ toxicity: heterophils have deeper cytoplasmic basophilia and partial degranulation
3+ toxicity: heterophils have deep cytoplasmic basophilia, moderate degranulation, abnormal granules and cytoplasmic vacuolisation
4+ toxicity: heterophils have deep cytoplasmic basophilia, moderate to marked degranulation with abnormal granules, cytoplasmic vacuolisation and karyorrhexis or karyolysis.

The number of toxic heterophils is graded as few (5-10%), moderate (11-30%) or marked (>30%).

Heterophils Learning Resources
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Test your knowledge using flashcard type questions
Avian Medicine Q&A 21
Reptiles and Amphibians Q&A 11


Thrall, M. (2004) Veterinary haematology and clinical chemistry Wiley-Blackwell

Fudge, A. (2000) Laboratory medicine: avian and exotic pets Elsevier Health Sciences

Feldman, B. (2000) Schalm's Veterinary haematology Wiley-Blackwell

Frye, FL & Williams, DL (1995) Self-Assessment Colour Review - Reptiles & Amphibians Manson

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