Key takeaways

• There are three granulocytes — neutrophils, eosinophils and basophils. They are also called polymorphonuclear leukocytes because of their multilobed nuclei.
• Mast cells are not granulocytes in the standard classification. They are granulated tissue-resident cells of a separate lineage with a single round nucleus, covered here because they pair functionally with basophils.
• Neutrophils dominate numerically and act by phagocytosis, reactive oxygen species and extracellular traps; the NADPH oxidase step is what fails in chronic granulomatous disease.
• Eosinophils target helminths and drive the late phase of allergic inflammation through major basic protein and eosinophil cationic protein.
• Basophils are the rarest leukocyte and, like mast cells, carry high-affinity IgE receptor and release histamine.
• Recruitment follows a fixed sequence — selectin-mediated rolling, integrin-mediated firm adhesion, then chemokine-directed transmigration.
• “Immature granulocytes” on a modern analyser is not the same as the band count: the IG parameter counts promyelocytes, myelocytes and metamyelocytes, and excludes bands.

Contents

• What granulocytes are
• Three granulocytes — and where mast cells fit
• Neutrophils
• Eosinophils
• Basophils
• Mast cells
• Surface markers for identification
• Development and maturation
• Activation and recruitment
• Immature granulocytes and left shift
• Granulocyte disorders
• Choosing reagents
• Frequently asked questions

Granulocyte reagents

Surface markers for identifying each population by flow cytometry, plus the granule proteins each cell type releases.

CD66b

Purified Anti-Human CD66b Antibody [G10F5]

PurifiedHuman

A granulocyte-restricted activation marker, widely used to identify and gate neutrophils.

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CD16

PerCP/Cyanine5.5 Anti-Human CD16 Antibody [3G8]

PerCP/Cy5.5Human

High on mature neutrophils and absent from eosinophils — the standard way to separate the two.

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MPO

Human Myeloperoxidase (MPO) ELISA Kit

Sandwich ELISAHuman

The defining azurophilic granule enzyme of neutrophils and a marker of degranulation.

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ECP

Human Eosinophil Cationic Protein (RNASE3) ELISA Kit

Sandwich ELISAHuman

An eosinophil-specific granule protein used as a marker of eosinophil activation in allergic disease.

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Tryptase

Human Tryptase ELISA Kit

Sandwich ELISAHuman

The principal mast cell protease; serum tryptase is the standard marker of mast cell activation.

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CD117 / KIT

PE Anti-Human CD117 Antibody [104D2]

PE conjugateHuman

KIT, the stem cell factor receptor — required for mast cell development and used to identify them.

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What granulocytes are

Granulocytes are white blood cells characterised by prominent cytoplasmic granules — membrane-bound vesicles packed with enzymes, antimicrobial peptides and inflammatory mediators. They are also called polymorphonuclear leukocytes, because their nuclei are divided into two or more connected lobes rather than being round.

That nuclear shape is the feature that separates them from the mononuclear leukocytes — lymphocytes and monocytes — and it is why a density gradient preparation of mononuclear cells specifically excludes them.

They are the fast arm of cellular immunity. Neutrophils in particular reach a site of infection within minutes to hours, long before an adaptive response can be mounted.

Three granulocytes — and where mast cells fit

Standard classification recognises three granulocytes, distinguished historically by how their granules take up histological stains.

Why mast cells sit outside the group

Mast cells are granulated and share important functions with basophils, so they are often discussed alongside granulocytes. They are nonetheless conventionally classified separately, for concrete reasons:

They are covered below because the functional pairing with basophils is genuine, but describing them as a fourth granulocyte conflates a shared granule phenotype with a shared lineage.

Neutrophils

The most abundant leukocyte and the first responder to bacterial and fungal infection. Neutrophils are short-lived, circulating for roughly eight hours before entering tissue, and the marrow produces them continuously in enormous numbers.

Their granules come in distinct classes: azurophilic (primary) granules carry myeloperoxidase, defensins and elastase; specific (secondary) granules carry lactoferrin; and gelatinase (tertiary) granules carry matrix metalloproteinases.

• Phagocytosis. Chemotaxis brings the cell to the site; bacteria are engulfed into phagosomes which fuse with granules to form phagolysosomes, where degradation occurs.
• Oxidative burst. NADPH oxidase generates superoxide, converted to hydrogen peroxide and, via myeloperoxidase, to hypochlorous acid. Loss of NADPH oxidase function causes chronic granulomatous disease, in which phagocytosis still occurs but killing fails.
• Neutrophil extracellular traps. In NETosis the cell releases decondensed chromatin studded with granule proteins, forming a web that immobilises bacteria. NETs also contribute to thrombosis and to autoimmune pathology, so the process is not purely protective.

Eosinophils

Eosinophils make up a few percent of circulating leukocytes but are far more numerous in tissue, particularly the gastrointestinal mucosa. Their characteristic bilobed nucleus and intensely eosinophilic granules make them easy to identify on a film.

• Antiparasitic action. Helminths are too large to phagocytose, so eosinophils degranulate onto the parasite surface, releasing major basic protein, eosinophil cationic protein, eosinophil peroxidase and eosinophil-derived neurotoxin.
• Allergic inflammation. Eosinophils dominate the late phase, arriving hours after the immediate mast cell response and sustaining inflammation in asthma, atopic dermatitis and eosinophilic oesophagitis.
• Tissue damage. The same cationic granule proteins that kill parasites damage host epithelium, which is a substantial part of why chronic eosinophilic inflammation remodels airways.

IL-5 is the principal growth and survival factor for eosinophils, which is why anti-IL-5 biologics are used in severe eosinophilic asthma.

Basophils

Basophils are the rarest leukocyte, typically well under one percent of the total. Their granules are so densely stained that they often obscure the nucleus entirely.

• IgE-mediated activation. Basophils carry the high-affinity IgE receptor FcεRI. Allergen cross-linking bound IgE triggers rapid degranulation.
• Mediator release. Histamine causes vasodilation and increased vascular permeability; leukotrienes and prostaglandins extend the response.
• IL-4 production. Basophils are an early and substantial source of IL-4, which reinforces Th2 polarisation — a role beyond simple effector degranulation.

Mast cells

Mast cells arise from haematopoietic progenitors but leave the marrow immature, completing differentiation in tissue under the influence of stem cell factor acting through KIT (CD117). They are concentrated at barrier surfaces — skin, airway and gut — and are long-lived.

• Immediate hypersensitivity. Like basophils they express FcεRI, and allergen cross-linking triggers release of pre-formed histamine, heparin and proteases within seconds. This is the mechanism of anaphylaxis.
• Tryptase is the dominant mast cell protease and, being relatively specific, serum tryptase is the standard laboratory marker of mast cell activation.
• Beyond allergy. Mast cells contribute to wound healing, angiogenesis, defence against venoms and parasites, and communication with the nervous system.

Clonal mast cell expansion causes mastocytosis, usually driven by activating KIT mutations — which is why CD117 is central both to their biology and to their identification.

Surface markers for identification

The two most useful discriminations in practice are CD16 for neutrophil versus eosinophil, and CD117 for mast cell versus basophil — pairs that are otherwise easy to confuse.

Development and maturation

Granulopoiesis proceeds in the bone marrow from the haematopoietic stem cell through the common myeloid progenitor and granulocyte-monocyte progenitor, then along a defined morphological sequence:

Myeloblast → promyelocyte → myelocyte → metamyelocyte → band cell → mature segmented granulocyte

Azurophilic granules appear at the promyelocyte stage and specific granules at the myelocyte stage, while the nucleus progressively indents and then segments. G-CSF drives neutrophil production and is used therapeutically to shorten chemotherapy-induced neutropenia; GM-CSF acts more broadly; IL-5 is specific to the eosinophil lineage.

Mast cell development diverges from this: committed progenitors leave the marrow and complete maturation in tissue, so mast cells never appear in the normal peripheral blood differential.

Activation and recruitment

Reaching a site of inflammation follows a fixed and well-characterised sequence:

The order matters: selectin-mediated rolling must precede integrin activation, which is why defects in either step — as in leukocyte adhesion deficiency — produce recurrent infection despite normal or high neutrophil counts.

Immature granulocytes and left shift

Under demand the marrow releases cells before full maturation, and this shows on a blood count. The terminology is worth getting right, because two related measures are frequently conflated.

• Band cells are the stage immediately before the mature segmented neutrophil, with an unsegmented horseshoe-shaped nucleus. A raised band count is the classic left shift.
• The immature granulocyte (IG) parameter reported by modern haematology analysers counts promyelocytes, myelocytes and metamyelocytes — and conventionally excludes band forms. It is therefore not a synonym for the band count, and the two are not interchangeable.

Either finding suggests the marrow is under pressure. Causes include bacterial infection and sepsis, systemic inflammation, tissue necrosis, corticosteroid or G-CSF administration, pregnancy, and marrow infiltration. A markedly raised IG count with other abnormalities warrants further investigation rather than being read in isolation.

Granulocyte disorders

Descriptions are for research context; interpretation of any blood count belongs with the treating clinician.

Frequently asked questions