The granulocytes are so called because they have densely granules in their cytoplasm; they are also called polymorphonuclear leukocytes because of their lobulated nuclei. There are three types of granulocytes:
- Neutrophils can perform phagocytosis and are the most abundant type of granulocytes and the most abundant type of white blood cells overall in most mammals.
- Eosinophils make up about 1–6% of white blood cells and play a role in defence against parasitic infections.
- Basophils are the least common of the granulocytes, they can perform phagocytosis, produce histamine and serotonin and these cells play a role in immune regulation and allergic responses.
Let us explore more details about these cells.
These cells are typically the first leukocytes to be recruited to an inflammatory site and are capable of eliminating pathogens by multiple mechanisms.
Super powers include:
- Release of neutrophil extracellular traps (NETs)
The picture of the blood smear above was taken from The Histology Guide. You can see the neutrophil has a single but multilobed nucleus stained in a pink color. The multilobed nuclei can have between 2 and 5 lobes.
Neutrophils are an important first line of defence and part of the innate immune system. They are crucial players in the response against fungal infection. The localization of neutrophils to the site of inflammation is important for clearance of the infection. Even though they play a big role in the innate immune response neutrophils can also regulate adaptive immune responses by releasing arginasea 1 and suppressing T cell proliferation and activity. Neutrophils can also act as antigen-presenting cells (APCs).
Eosinophils contribute to the pathogenesis of different diseases, including asthma and primary hypereosinophilic syndromes. They are activated and recruited into tissues in response to inflammatory stimuli.
Super powers include:
- Fighting parasitic infections
- Produce reactive oxygen species such as hypobromite, superoxide, and peroxide
The picture of the blood smear above was taken from The Histology Guide and shows an eosinophil with a two lobed nucleus. Eosinophils have large acidophilic specific granules that are stained bright red, or reddish-purple.
Eosinophils play a role in host defence against parasites such as helminths and can elicit a profound TH2-type cytokine-mediated pathology. In the gif below mouse eosinophils are migrating toward a C. elegans dauer larva, a parasite.
Like eosinophils, basophils play a role in both parasitic infections and allergies. Basophils have protein receptors on their cell surface that bind IgE, an immunoglobulin involved in parasite defense and allergy.
Super powers include:
- Production of heparin, which prevents blood from clotting too quickly and histamine, which promotes blood flow to tissues
- Fighting ectoparasite infection, e.g., ticks
The picture of the blood smear above was taken from The Histology Guide and shows a a basophil with granules stained in deep blue. The contents of the granules include heparin, histamine and serotonin.
A review by Nature Immunology goes in more detail about the various roles basophils. Superpowers:
- Initiating of chronic allergy
- IgG-mediated anaphylaxis
- Driving TH2-cell differentiation
Where do Mast Cells fit in?
Just like granulocytes, mast cells have prominent dense granules in their cytoplasm and the contents of these granules include histamine, heparin and proteases. Event hough mast cells look morphologically very similar to basophils and both express CD34, mast cells have a different bone marrow precursors. Below is a diagram from Nature Reviews Immunology 12, 253-268 (April 2012), showing the development of neutrophils, eosinophils, basophils and mast cells and how they originate from different precursors.
Unlike basophils, masts cell circulate in an immature form, maturing only in tissue sites where they are resident. They are long-lived cells, able to survive for months or years. At the earliest stages of infection, mast cells are important for communicating the presence of a pathogen to many cell types located nearby. Mast cells are well known to have a role in allergy and anaphylaxis, but are also found to be involved in wound healing, angiogenesis and immune tolerance.
Mast cells express a high-affinity receptor (FcεRI) for the Fc region of IgE, the least-abundant member of the antibodies and as a result mast cells are coated with IgE. When they encounter the antigen specific for the antibody mast cells are stimulated to release the contents of their granules (process known as degranulation) which is what causes the symptoms of allergy.
One of the chemicals mast cells release is histamine. Histamine is responsible for dilation of post-capillary venules, increase in blood vessel permeability which then leads to swelling and redness.
- New tools for studying the in vivo function of basophils
- Granulocyte isolation reagent to obtain human neutrophils, eosinophils and basophils from whole blood
The Histology Guide: White blood cells
Newly discovered roles for basophils: a neglected minority gains new respect. Nature Reviews Immunology 9, 9-13 (January 2009).
Neutrophil recruitment and function in health and inflammation. Nature Reviews Immunology 9, 9-13 (January 2009)
Eosinophils: changing perspectives in health and disease. Nature Reviews Immunology 13, 9-22 (January 2013).
Coordinated regulation of myeloid cells by tumours. Nature Reviews Immunology 12, 253-268 (April 2012)
Immunobiology: The Immune System in Health and Disease. 5th edition. Janeway CA Jr, Travers P, Walport M, et al. New York: Garland Science; 2001.