What are Granulocytes?

Betsy Chesnutt, Artem Cheprasov
  • Author
    Betsy Chesnutt

    Betsy has a Ph.D. in biomedical engineering from the University of Memphis, M.S. from the University of Virginia, and B.S. from Mississippi State University. She has over 10 years of experience developing STEM curriculum and teaching physics, engineering, and biology.

  • Instructor
    Artem Cheprasov

    Artem has a doctor of veterinary medicine degree.

Learn about granulocytes, including learning what are granulocytes with the granulocytes definition. Additionally, learn the function and types of granulocytes. Updated: 08/15/2021

Table of Contents


What Are Granulocytes and Granulocytic Cells?

Dangerous pathogenic bacteria and viruses are always trying to infect the human body, where they can cause serious illness. However, the body has some pretty strong defenses that kill these invading pathogens and prevent and fight infections. White blood cells circulate in the bloodstream, always on the lookout for signs of infection. If they find any microorganisms that shouldn't be there, they respond by attacking and killing the invading pathogens.

One of the strongest defenses is provided by a special group of white blood cells known as granulocytes. These cells are leukocytes that are part of the innate immune system, which means that they can quickly respond and destroy all kinds of microorganisms, including bacteria, viruses, fungi, and parasites. Granulocytes are large white blood cells that contain small particles called granules. These granules are full of enzymes that can be released to destroy microorganisms and fight infections. Because these enzymes have a variety of effects on the body, granulocytes also play a role in inflammatory disorders such as allergic reactions and asthma.

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Granulocytes Definition

A granulocyte is a type of white blood cell that contains small particles called granules, which contain enzymes that help to kill pathogens and fight infections. There are three types of granulocytes: neutrophils, basophils, and eosinophils. All three types of granulocytes contain a large, multi-lobed nucleus, and so granulocytes are also known as polymorphonuclear leukocytes.

Both neutrophils and eosinophils are granulocytes that are part of the innate immune system

Image of two types of granulocytes: eosinophils and neutrophils

What Do Granulocytes Do?

Granulocytes circulate in the blood looking for chemical signals that alert them to the presence of microorganisms. These chemical signals can be on the surface of or released by harmful microorganisms that are trying to invade the body. When a granulocyte detects the presence of a pathogen, it will leave the blood and travel into the affected tissue. This process is known as chemotaxis. When a granulocyte encounters a pathogen, it will release the contents of its granules. The granules contain enzymes that incapacitate and kill the invading microorganisms. Granulocytes respond in a non-specific way to any kind of infection, so they are one of the first defenses protecting the body from infection.

The enzymes released by granulocytes can have other effects on the body as well. Some enzymes increase inflammation, which also helps to recruit more white blood cells and can lead to the development of other inflammatory responses like fever and swelling. Since many microorganisms only reproduce at a specific temperature, the presence of a fever can make it easier for the immune system to kill them and stop the infection. However, in some cases, granulocytes make mistakes and respond to other harmless substances, such as pollen or dust mites. When inflammation occurs in the absence of an infection, it can lead to autoimmune disorders such as allergic reactions and asthma.

Types of Granulocytes

There are three types of granulocytes: neutrophils, basophils, and eosinophils. All three cells look very similar, but they contain different combinations of enzymes in their granules.

Neutrophils and Neutrophil Granulocytes Function

Neutrophils are the most common type of white blood cell found in the blood, making up about 55-70% of the total number of white blood cells in healthy people. Neutrophils are constantly being produced by the bone marrow because they are very short-lived, usually lasting less than a day after reaching maturity and being released into the circulating bloodstream. Neutrophils are a critical part of the immune system because they are the first cells to respond to an infection or injury, typically arriving at the site in about 30 minutes.

To get out of the blood vessels and into the infected tissues, neutrophils release some of the enzymes in their granules to dissolve the bonds that hold cells together. This allows them to crawl through the blood vessel walls and into the tissue, where they can begin attacking invading microorganisms.

Activated neutrophils secrete enzymes to dissolve junctions between cells so they can enter tissues and destroy micro-organisms.

A diagram showing the process of neutrophil activation and phagocytosis of bacteria

Neutrophils have several weapons that they use to fight infections. First, they are phagocytic cells that destroy microorganisms by engulfing them. In addition to phagocytosis, neutrophils also destroy pathogens by releasing enzymes that break down bacterial cell walls and generate compounds such as hydrogen peroxide that are toxic to many microorganisms. They can also create webs of proteins that trap and destroy microorganisms. Once a neutrophil leaves the bloodstream, it will not return. It will eventually turn into a pus cell, and the presence of many neutrophils is what causes the pus that seeps out of infected wounds.

Basophils and Basophil Granulocyte Function

Basophils are much less abundant than neutrophils, making up less than 1% of the total number of white blood cells. Like neutrophils, they are short-lived cells that only persist for about a day. The granules in basophils contain a protein called histamine. Histamine is primarily responsible for promoting inflammation. It causes blood vessels to become leaky, which results in swelling and redness in the affected area. Basophils and the histamine they release are responsible for many of the symptoms of allergic reactions, where tissues swell and become inflamed in response to non-pathogenic substances that the immune system thinks might be harmful. To reduce the effects of histamine, people with allergies typically take anti-histamine medications that bind to and neutralize histamine.

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Frequently Asked Questions

What is the function of granulocytes?

Granulocytes are white blood cells that are part of the innate immune system, which means that they respond quickly to any type of infection or injury, usually within about 30 minutes. They contain granules that release enzymes to kill or neutralize microorganisms and toxins, so they play a big role in protecting the body from infection.

What are the 3 types of granulocytes and their functions?

The three types of granulocytes are neutrophils, basophils, and eosinophils. Neutrophils are the most abundant (up to 70% of total white blood cells), and they kill and eat pathogens like bacteria and viruses. Basophils are the least abundant (less than 1% of total white blood cells) and their primary function is to release histamine, which contributes to the inflammatory response and recruits more white blood cells to the site of the infection.

What is the normal range of granulocytes?

Neutrophils usually make up 55-70% of the total number of white blood cells in the blood. Eosinophils make up 1-5% of the total, and basophils are less than 1% of the total number of white blood cells.

What does it mean when your granulocytes are high?

Neutrophils can be increased when you have an acute infection, or when you have a chronic inflammatory disease like rheumatoid arthritis. They are also usually increased in people who smoke. Basophils and eosinophils are usually higher in people with allergies. The number of all kinds of granulocytes can also increase because of cancers like leukemia.

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