[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [General pathology](https://durgical.com/en/general-pathology/) › [White cells, lymph nodes, spleen and thymus](https://durgical.com/en/general-pathology/areas/hematolymphoid/) › Reactive leukocytosis

# Reactive leukocytosis

- Lesion

A high white count is usually the body reacting to infection, allergy or stress. Which cell rises points to the cause.

Updated Nov 12, 2026

## Key points

- Neutrophilia with left shift, toxic granules and Döhle bodies points to bacterial infection or sepsis.
- Eosinophilia: allergy, asthma, drug reactions, helminths, some vasculitides and Hodgkin lymphoma.
- Atypical (reactive) lymphocytes in infectious mononucleosis are activated CD8 T cells, not the EBV-infected B cells.
- Leukaemoid reaction vs CML: CML has basophilia and the BCR::ABL1 fusion; a reactive picture lacks both.
- Basophilia is rare and suggests a myeloproliferative neoplasm.

## What it is

Leukocytosis is common and usually reactive. Each type of white cell rises in its own settings because different cytokines drive different lineages: bacteria and tissue necrosis raise neutrophils, parasites and allergy raise eosinophils, viruses raise lymphocytes, and chronic infections raise monocytes. The main pitfalls are a leukaemoid reaction, which can look like chronic myeloid leukaemia, and the atypical lymphocytes of viral infection, which can look like lymphoma or leukaemia.

## Structure

- Toxic granulation: coarse, dark azurophilic granules in neutrophils during severe infection.
- Döhle bodies: pale blue cytoplasmic patches of dilated endoplasmic reticulum.
- Atypical lymphocytes: large cells with abundant blue cytoplasm that seems to wrap around neighbouring red cells.

## Step by step

- An infection, allergen or necrosis triggers macrophages, T cells and stromal cells to release cytokines.
- IL-1 and TNF release mature neutrophils from the marrow pool; colony-stimulating factors speed production.
- IL-5 drives eosinophils; T-cell responses to viruses expand lymphocytes.
- Immature granulocytes enter the blood (left shift); the count falls once the stimulus is controlled.

## Reactive leukocytosis: the cell that rises and its usual causes

| Pattern | Main drivers | Typical causes | Clue on the smear |
| --- | --- | --- | --- |
| Neutrophilia | IL-1, TNF, G-CSF | Bacterial infection, sepsis, burns, infarction, glucocorticoids | Left shift, toxic granules, Döhle bodies |
| [Lymphocytosis with atypical cells](https://durgical.com/en/conditions/infectious-mononucleosis/) | Antiviral T-cell response | Infectious mononucleosis, cytomegalovirus, viral hepatitis, pertussis | Large reactive lymphocytes with abundant cytoplasm |
| [Eosinophilia](https://durgical.com/en/conditions/hypereosinophilic-syndrome/) | IL-5 | Allergy, asthma, drug reactions, helminths, some vasculitis and Hodgkin lymphoma | Bilobed cells with orange granules |
| Basophilia | Unclear | Rare when reactive; points to a myeloproliferative neoplasm | Dark purple granules hiding the nucleus |
| [Monocytosis](https://durgical.com/en/conditions/tuberculosis/) | Chronic cytokine drive | Tuberculosis, endocarditis, brucellosis, inflammatory bowel disease, lupus | More monocytes with folded nuclei |
| [Leukaemoid reaction](https://durgical.com/en/conditions/chronic-myeloid-leukaemia/) | Extreme cytokine drive | Severe infection, some tumours; the main mimic of CML | Very high count with immature granulocytes, no basophilia, no BCR::ABL1 |

## Clinical notes

- Mononucleosis: fever, sore throat, lymphadenopathy and splenomegaly with atypical lymphocytes and a heterophile antibody.
- Persistent unexplained eosinophilia raises the question of parasites in Colombia and Latin America, and later of a clonal disorder.

## Memory aid

Bugs bring neutrophils, worms bring eosinophils, viruses bring lymphocytes; basophils bring a haematologist.

## Test yourself

Think of the answer, then tap the question to see it.

## Which cells are the atypical lymphocytes of infectious mononucleosis?

Mostly activated CD8 cytotoxic T cells reacting against EBV-infected B cells.

## Which two findings separate chronic myeloid leukaemia from a leukaemoid reaction?

Basophilia and the BCR::ABL1 fusion (Philadelphia chromosome) favour CML.

## What are Döhle bodies?

Pale blue patches of dilated endoplasmic reticulum in neutrophil cytoplasm, a sign of severe infection.

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## Elsewhere in DurgiPedia

- [White blood cells: types and functions](https://durgical.com/en/physiology/white-blood-cells-types-and-functions/)
- [Eosinophil](https://durgical.com/en/immunology/eosinophil/)
- [Neutrophil](https://durgical.com/en/immunology/neutrophil/)
- [Mast cell and basophil](https://durgical.com/en/immunology/mast-cell-and-basophil/)
- [Monocyte and macrophage](https://durgical.com/en/immunology/monocyte-and-macrophage/)

## Connections

## Related 4

- [Systemic effects of inflammation: fever, acute-phase proteins, leukocytosis, sepsis](https://durgical.com/en/general-pathology/systemic-effects-of-inflammation-fever-acute-phase-proteins-leukocytosis-sepsis/)
- [Chronic myeloid leukaemia](https://durgical.com/en/general-pathology/chronic-myeloid-leukaemia/)
- [Reactive lymphadenitis: the patterns](https://durgical.com/en/general-pathology/reactive-lymphadenitis-the-patterns/)
- [Intestinal helminths](https://durgical.com/en/general-pathology/intestinal-helminths/)

## Related conditions 7

- [Infectious mononucleosis](https://durgical.com/en/conditions/infectious-mononucleosis/)
- [Sepsis and septic shock](https://durgical.com/en/conditions/sepsis-and-septic-shock/)
- [Hypereosinophilic syndrome](https://durgical.com/en/conditions/hypereosinophilic-syndrome/)
- [Chronic myeloid leukaemia](https://durgical.com/en/conditions/chronic-myeloid-leukaemia/)
- [Cytomegalovirus infection](https://durgical.com/en/conditions/cytomegalovirus-infection/)
- [Tuberculosis](https://durgical.com/en/conditions/tuberculosis/)
- [Eosinophilic pneumonias (including Löffler syndrome)](https://durgical.com/en/conditions/eosinophilic-pneumonias/)

## Microorganisms 1

- [Epstein–Barr virus](https://durgical.com/en/microbiology/epstein-barr-virus/)

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Educational content on the mechanisms of disease; not a diagnostic or treatment guide and no substitute for a health professional's advice. No doses are given.

Sources: [Chronic Myeloid Leukemia Treatment (PDQ®)–Patient Version](https://www.cancer.gov/types/leukemia/patient/cml-treatment-pdq) (National Cancer Institute (NCI), 2026).

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Representative model, for educational purposes only.

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