Reactive leukocytosis
- Lesion
A high white count is usually the body reacting to infection, allergy or stress. Which cell rises points to the cause.
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 | Antiviral T-cell response | Infectious mononucleosis, cytomegalovirus, viral hepatitis, pertussis | Large reactive lymphocytes with abundant cytoplasm |
| Eosinophilia | 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 | Chronic cytokine drive | Tuberculosis, endocarditis, brucellosis, inflammatory bowel disease, lupus | More monocytes with folded nuclei |
| Leukaemoid reaction | 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.
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.
Elsewhere in DurgiPedia
- White blood cells: types and functions
- Eosinophil
- Neutrophil
- Mast cell and basophil
- Monocyte and macrophage
Connections
Microorganisms1
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 (National Cancer Institute (NCI), 2026).
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Representative model, for educational purposes only.