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What are Granulocytes?

A type of white blood cells in the immune system, granulocytes are differentiated by the presence of a particular granule in their cytoplasm. As a result of their varying nuclei shapes, they are known as polymorphonuclear leukocytes, typically lobed into three sections. Their site of production is the bone marrow, produced through the process of granulopoiesis. These granulocytes are phagocytes which can ingest foreign particles such as viruses, bacteria and other such parasites. They get their name from the granules of enzymes that they possess which enables them to digest the microbes that have invaded. Approximately 60% of the white blood cells constitute granulocytes.

Granulocytes have a critical role – to wade off infections. Their distinctive morphology is in possessing large cytoplasmic granules (which are stained by basic dyes) and a nucleus (bi-lobed). Usually, granulocytes are pivotal in both innate and adaptive immune responses in their attack against infections caused by microbes. Granulocytes, as part of immune response, migrate to the infection-site and release a good quantity of various effector molecules such as cytokines, histamine, chemokines, enzymes and other growth factors. Consequently these structures are essential parts of inflammation and have a role to play in the etiology of allergies.

There are numerous of these cells and their diameter is close to 12-16 micrometres, hence larger than erythrocytes (Red blood cells). They are crucial as they serve as mediators of the inflammatory response.

Site of development

Granulocytes are produced through the process of granulopoiesis in the bone marrow from the stem cells. The differentiation of these cells from the pluripotent hematopoietic stem cells into granulocytes is granulopoiesis. Differentiation includes cells such as promyelocytes and myeloblasts.

Granulocytes – Types

Granulocytes are of four different types, each of this type except for mast cells, can be identified by the colour that granules stain when encountered with a compound dye. The contrast in staining features depicts the differences in the chemical composition of the granules.

Different types of granulocytes –

1. Basophils

It accounts for close to 0.5% of circulating blood leukocytes and is the least common type of granulocyte. It participates in many roles such as stimulation and differentiation of CD4+ cells, antigen presentation etc. Their nucleus is lobed having two lobes where the chromatin filaments connecting them are not very distinct.

2. Eosinophils

These account for close to 1% of the circulating leukocytes. The role of these structures vary in the immune responses and in the pathogenesis of autoimmune or allergic diseases. The granule count in them can vary as a result of their tendency to degranulate in the bloodstream. They have a kidney shaped nucleus that is lobed. Their granules contain a unique protein that is crucial for killing parasites.

3. Neutrophils

Amongst all other types, these are the most abundantly found leukocytes in the bodies of humans. It forms the vanguard of the cellular immune response of the body. Found commonly in the bloodstream, these types of phagocytes account for 60-65% of the total circulating white blood cells. These, in turn, have two components – neutrophil-killers and neutrophil-cagers. Neutrophils, once upon receiving suitable signals, leave the blood to arrive at the site of infection. They do not revert to blood but turn into pus cells and die. Until they are mature, the neutrophils do not leave the bone marrow normally. However, when there is an infection, neutrophil precursors known as promyelocytes and myelocytes are liberated.

4. Mast cells

These types of granulocytes are found in tissues wherein the granules are richly supplied with histamine and heparin. Their role is pivotal in activities related to immune tolerance, allergies to response to pathogens (example – parasites). They mediate autoimmunity and inflammation along with moderating and regulating neuroimmune system responses.

Functions of Granulocytes

Predominantly, granulocytes are defensive in nature against the invading microbes. The cellular makeup of these cells makes them appropriate for this role. They are recruited from the bone marrow whenever they are instructed to and burgeons from the progenitor cells after an infection. These structures are extremely mobile and have the ability to invade every tissue of the entity. They exhibit positive chemotaxis, the form of sensitivity is critical to the destruction of microbes.
These secretory cells are highly phagocytic cells containing microbicidal systems varying digestive enzymes. Granulocytes are most likely credited for amplification of inflammatory events.

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Frequently Asked Questions

What is the main function of granulocytes?

The main function of granulocytes is to provide protection against microbes and allergens.

What are agranulocytes and how do they differ from granulocytes?

Agranulocytes are white blood cells that do not have granules. The agranulocytes have a single lobe in their nucleus while a granulocyte nucleus is usually 2-5 lobed.

What do low granulocytes mean?

Low granulocytes cause a condition called granulocytopenia where the concentration of the cells go down to as low as 500/mm3. A person with this condition is highly sensitive to infections.

What do granulocytes release?

As a part of their immune response, the granulocytes release a number of effector molecules such as cytokines, histamines, chemokines, growth factors and enzymes.

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