White blood cells, also known as leukocytes, are immune cells that circulate in the blood and lymphatic system.

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There are 5 main types:

Neutrophils – main action against bacterial and fungal infections.Monocytes – main action against bacterial infections.Eosinophils – main action against parasitic infections.Basophils – responsible for responses to allergens.Lymphocytes – main action against viral infections.

Neutrophils, eosinophils, and basophils are granulocytes since they have cytoplasmic granules which can digest microorganisms.

Lymphocytes and monocytes are agranulocytes since they lack granules in their cytoplasm.

This article will give a basic overview of the five main types of white blood cells. More detailed information is available in separate articles.


Neutrophils are granular leukocytes and develop from the myeloid cell lineage within the bone marrow. They are the most abundant leukocyte type, making up 40-70% of those found in peripheral blood. Neutrophils are primarily involved in the immune response against bacterial infections and their presence in tissue is associated with acute inflammation.

They are 9-16µm in diameter and have a multilobed nucleus. Their cytoplasm contains granules with degradative enzymes which are released during phagocytosis. They have a short lifespan of 2-3 days and are one of the first responders to invading microbes.

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Fig 1 – Blood film showing neutrophils with multi-lobed nuclei

Neutrophils are part of the innate immune system and are the first cells to arrive at the site of infection. In response to to damage or pathogens, tissues produce chemokines which recruit neutrophils in a process called chemotaxis, allowing neutrophils to hone in to the site of infection.

Here, the neutrophils phagocytose microorganisms and subsequently digest them by releasing granules into the phagosome. Neutrophils can also release substances to attract monocytes which, upon entering the tissue, differentiate to macrophages.

Bacterial infections, stress, cancers (e.g. leukaemia) and trauma can cause high neutrophil counts.

Chemotherapy, aplastic anaemia, Vitamin B12 deficiency and cancer (e.g. myelodysplasia) can cause low neutrophil counts. 

Note: some labs may report neutrophil count as PMN (polymorphonuclear neutrophils).

Further information on neutrophils and their function can be found in our articles on phagocytes, phagocytosis and acute inflammation.


Monocytes belong to the myeloid cell line within the bone marrow. They are primarily involved in the immune response against bacterial infection and make up roughly 5-10% of all circulating leukocytes. They have a kidney bean-shaped nucleus.

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Fig 2 – Section of a blood film showing a monocyte with a kidney-bean shaped nucleus

Monocytes are circulating leukocytes which typically remain in the blood for around 8 hours before migrating into tissue where they differentiate into macrophages.

Macrophages then form the main population of phagocytic cells within tissues and have a much longer lifespan than neutrophils, lasting months or even years. In some tissues, resident macrophages have specific names e.g. Kupffer cells in the liver and osteoclasts in the bone. They are much larger than neutrophils, with a diameter of 25-50µm and have a single-lobed, round nucleus.

Macrophages then phagocytose microorganisms and digest them by releasing granules into the phagosome. They also secrete cytokines which modulate the immune response.

In certain situations, monocytes can also differentiate into dendritic cells. These form an important link between the innate and adaptive immune systems. They assist in T cell activation during the adaptive immune response and are the only cell type that can activate naïve T cells.

Further information on monocytes and their function can be found in our articles on phagocytes and phagocytosis.


Fig 3 – Blood film showing eosinophils, note the high uptake of eosin pigment in the cytoplasm


Basophils are granular leukocytes of myeloid lineage. They are 14-16µm in diameter and have a bi-lobed, S-shaped nucleus. Basophils circulate through the peripheral blood and have a lifespan of roughly 2 weeks.

They are very similar in function and appearance to mast cells, which are found within tissues.

Basophils contain histamine granules and cause local inflammatory responses through their interaction with IgE. Their role in the immune system is poorly understood but they potentially mediate type I hypersensitivity reactions alongside mast cells. Examples include asthma and anaphylaxis.

Patients with allergic reactions typically have high basophil counts.

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Fig 4 – Section of a blood film showing a basophil


Lymphocytes are agranular leukocytes which form from the lymphoid cell line within the bone marrow. They respond to viral infections and are the smallest leukocytes, with a diameter of 6-15µm. Lymphocytes have round, densely-staining nuclei with sparse cytoplasm.

They circulate between the tissues, peripheral blood and lymphatic system. Their lifespan also varies depending on the subtype that they differentiate into.

There are three major types of lymphocytes:

Natural killer cellsT cellsB cells
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Fig 5 – Blood film from a patient with Chronic Lymphocytic Leukaemia showing multiple lymphocytes

Natural Killer Cells

NK cells provide non-specific immunity against cells displaying foreign proteins such as cancer cells and virally- infected cells. They make up less than 5% of circulating leukocytes.

Although they function more effectively alongside T helper cells, they can independently detect and kill pathogens as part of the innate immune system.

NK cells can detect abnormal cells and, in response, release perforins. Perforins are cytolytic proteins which embed into the plasma membrane. This creates channels which allow extracellular fluid into the cells, causing them to burst (lysis).

T Cells

T cells form in the bone marrow but mature in the thymus. They are part of the adaptive immune system and are involved in cell-mediated immunity.

Once active, Cytotoxic T cells can directly attack infected cells. In addition, helper T cells have many functions including activating B cells and forming memory T cells which respond on re-infection.

Further information can be found in our articles on T cells and T cell memory.

B Cells

B cells form and mature in the bone marrow. They are part of the adaptive immune system and involved in humoral immunity by secreting antibodies. Once active, B cells mature into plasma cells which secrete antibodies, and memory B cells.

Further information on B cells can be found here.

Clinical Relevance – Eosinophilic Oesophagitis

Eosinophilic oesophagitis (EO) is a chronic, inflammatory, immune disease. Eosinophils infiltrate the epithelial lining of the oesophagus and mediate an allergic response against ingested food. Elevated eosinophil numbers and immune activation then leads to inflammation and tissue injury. EO causes similar symptoms to gastro-oesophageal reflux disease (GORD) but typically does not respond to proton pump inhibitors.

Patients typically present with epigastric pain, dysphagia and food bolus obstruction. They also, commonly, have a history of allergic diseases such as asthma, rhinoconjunctivitis and oral allergy syndrome.

Diagnosis is by identifying eosinophilic infiltration on biopsy.

The mainstay of treatment is topical steroids.

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Patients use steroid inhalers. However, the dose is sprayed into the mouth and dry-swallowed rather than inhaled.

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Fig 6 – Tissue biopsy from a patient with eosinophilic oesophagitis showing infiltrates containing eosinophils