Leucocytes, also known as white blood cells, are immune cells that circulate the blood and lymphatic system.
There are 5 main types of leucocytes
- Neutrophil – main action against bacterial and fungal
- Monocyte – main action against bacterial
- Eosinophil – main action against parasitic
- Basophil – responsible for allergic
- Lymphocyte – main action against viral
Neutrophils, eosinophils, and basophils are also known as granulocytes since they contain granules that can digest microorganisms.
Lymphocytes and monocytes are known as agranulocytes since they lack granules in their cytoplasm
This article will give a basic overview of the 5 main types of leucocyte. More detailed information will be available in separate articles.
Neutrophils are granular leucocytes formed from the myeloid cell line within the bone marrow. They are the most common leucocyte, making up around 40-70% of those found in peripheral blood. They are primarily involved in the immune response against bacterial infection and their presence in tissue is associated with acute inflammation.
They are typically 9-16µm in diameter and have a multi–lobed nucleus. The cytoplasm contains granules which contain degradative enzymes, which are released during phagocytosis. They have a relatively short lifespan of 2-3 days.
Neutrophils are part of the innate immune system and are the first cell types to travel to the site of an infection via chemotaxis since the chemoattractive substance that attracts neutrophils is the first to be produced by tissues.
Here the neutrophils phagocytose microorganisms and subsequently digest them by releasing granules into the phagosome. Following this, the neutrophil will release substances that attract monocytes which, when they enter tissue, become macrophages.
A high neutrophil count is often seen in bacterial infection.
Note: some labs may report neutrophil count as PMN (polymorphonuclear neutrophils).
Monocytes are formed from 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 circulating leucocytes. They have a kidney bean shaped nucleus.
Monocytes are circulating leucocytes 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 of months or even years. In some tissues the resident macrophages have specific names e.g. Kuppfer cells in the liver and osteoclasts in the bone. They are also much larger than neutrophils, with a diameter of 25-50µm and have a single-lobed, round nucleus.
They then phagocytose microorganisms and digest them by releasing granules into the phagosome. They also secrete cytokines to modulate the immune response.
If certain situations monocytes can also differentiate into dendritic cells, which 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.
Eosinophils, named so because they histologically stain with eosin, are granular leucocytes formed from the myeloid cell line within the bone marrow. They typically make up 1-3% of circulating leucocytes and are mainly found within tissues, generally only spending around an hour in peripheral blood.
They have a diameter of 12-17µm and have a bi-lobed, sausage shaped nucleus.
Their granules contain molecules such as major basic protein (MBP), cationic protein and peroxidase that are toxic to parasites. They also phagocytose antigen-antibody complexes.
A high eosinophil count is typically seen in patients with parasitic infections, allergic reactions, and in some autoimmune diseases.
Basophils are granular leucocytes formed from the myeloid cell line within the bone marrow. They are 14-16µm in diameter and have a bi-lobed, S-shaped nucleus. These cells are found in 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 granules containing histamine and they cause local inflammatory responses through interactions with IgE. Their role in the immune system is poorly understood but it is thought that they mediate type I hypersensitivity reactions alongside mast cells, such as asthma and anaphylaxis.
A high basophil count is typically seen in patients with allergic reactions.
Lymphocytes are agranular leucocytes formed from the lymphoid cell line within the bone marrow. They are primarily involved in the immune response against viral infection. These are the smallest leucocytes, with a diameter of 6-15µm. They have round, densely staining nuclei and sparse cytoplasm.
They circulate between tissues, peripheral blood and the lymphatic system. Their lifespan also varies depending on the subtype that they differentiate into.
There are three major types of lymphocytes:
- Natural killer cells
- T cells
- B cells
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 leucocytes.
Although they function more effectively alongside T helper cells, they are able to detect and kill pathogens independently as part of the innate immune system.
Once an abnormal cell is detected, NK cells release perforins which embed into the plasma membrane and create channels that allow extracellular fluid to enter the cells causing them to burst (lysis).
T cells are formed in the bone marrow but mature in the thymus.
T cells are part of the adaptive immune system and involved in cell-mediated immunity via its T-cell receptor.
Once active, helper T cells activate B cells, cytotoxic T cells directly attack infected cells, and memory T cells are formed which are responsible for mounting an immune response to reinfection.
Further information can be found in our articles on T cells and T cell memory.
B cells are formed and mature in the bone marrow.
B cells 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 is a disease of the oesophagus caused by infiltration of eosinophils within the epithelium. It 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 identification of eosinophilic infiltration on biopsy.
The mainstay of treatment is topical steroids. A steroid inhaler is typically used, but the dose is sprayed into the mouth and dry swallowed rather than inhaled.