Neutrophils: Definition & Function

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  • 0:01 What Are Neutrophils?
  • 0:27 Neutrophil Morphology
  • 0:50 Granules
  • 1:21 Neutrophil Functions
  • 2:22 Phagocytosis
  • 2:54 NETs
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Lesson Transcript
Instructor: Katy Metzler

Katy teaches biology at the college level and did her Ph.D. work on infectious diseases and immunology.

Even though there are germs all around us all the time, we rarely get sick. That's because our immune system is constantly on guard, protecting us from infections. In this lesson, learn about neutrophils, the first responders of the immune system.

What Are Neutrophils?

Our innate immune system is the part of our immune system that can already function as soon as we are born, and doesn't need to develop over our lifetime like the adaptive immune system does.

Neutrophils are white blood cells that play some very important roles in our innate immune system. They circulate around our body in the bloodstream, and when they sense signals that an infection is present, they are the first cells to migrate to the site of the infection to begin killing the invading microbes.

Neutrophil Morphology

Neutrophils are also called 'polymorphonuclear cells.' This big, intimidating word just means that the neutrophil nucleus has a complex, lobulated shape. In a cross-section, neutrophils may even look like they have more than one nucleus. Scientists still don't know why neutrophils have this unique nuclear shape, but it certainly makes these cells very easy to identify under the microscope!



Neutrophils belong to a class of innate immune cells called granulocytes. Granulocytes get their name from the many granules in their cytoplasm. Granules are small, membrane-bound vesicles that contain various types of contents.

In neutrophils, there are three different types of granules that help the cells perform their functions. Gelatinase granules help neutrophils exit the bloodstream and migrate through tissues to get to the site of infection. Azurophilic and specific granules contain antimicrobial proteins that allow neutrophils to kill microbes.

Neutrophil Functions

Neutrophils' main job is to kill as many germs as possible. You can imagine neutrophils as the soldiers of the innate immune system.

First, they are very abundant, making up about 60% of the immune cells in our blood! This means that there are plenty of them around to respond to microbial invaders.

Second, neutrophils are heavily armed. Their granules contain antimicrobial effectors, which you can imagine as our body's natural antibiotics. These antimicrobial proteins can damage and kill microbes in various different ways.

Third, neutrophils go to the front lines. Neutrophils are the first cells to migrate to a site of infection, and with the help of other innate immune cells, they work hard to keep things under control until the powerful adaptive immune system kicks in, a process that can take days to weeks.

Finally, neutrophils are prepared to die for their cause. Each neutrophil only lives for a matter of hours, and new neutrophils are constantly being produced in our bone marrow. This means that these cells can use a dramatic suicide mechanism to control microbes. Let's talk about how that works.


How do neutrophils kill microbes? One major strategy is phagocytosis, where neutrophils engulf microbes into intracellular pouches called 'phagosomes.' Phagocytosis is kind of like when you engulf dog droppings into a plastic bag; the nasty stuff stays inside the sealed pouch.

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