Neutrophils - Cells of the Immune System

The immune system is the body’s main defence against infection. In order to function properly, the immune system must be able to detect and protect against infinite agents such as pathogens including viruses and bacteria and unhealthy or infected cells. In order to do this, various cells are required to carry out specific functions. In the article below, an overview of neutrophils will be discussed including their function and development.

What is a Neutrophil?

Neutrophils are the most abundant white blood cell in the circulation are regarded as the first line of defence of the innate immune system. They are characterised by their multi-lobed shape of their nucleus (Rosales, 2018). They are produced in the bone marrow and migrate through the lymphatic system to different infected tissues. Neutrophils are recruited to the site of infection via molecules called chemokines via chemotaxis which provide signals to cells to direct them to sites of inflammation (Rosales, 2018). Neutrophils have various cell surface receptors that allow them to detect chemical gradients of molecules such as IFN-γ IL-8, C3a, C5a and Lukotriene B4 (LTB4) (Wikipedia Contributors, 2020).

Neutrophil Functions

Neutrophils survey the host to identify infection. Neutrophils aid as the first line of defence of the innate immune system by phagocytosis of pathogens via the interaction of PRRs and PAMPs and/or the release of granules containing antimicrobial factors and production of reactive oxygen species which are damaging to the pathogen (Mayadas et al., 2014). The granules within neutrophils contain enzymes such as granzymes which aid in breaking down the cell wall of the pathogen, thus causing it to lyse and die. Additionally, neutrophils produce NETs (neutrophil extracellular traps) which contain various enzymes such as serine proteases and chromatin to trap and kill extracellular microbes (Mayadas et al., 2014).  

Furthermore, neutrophils play a role in inflammation and immune responses based on the infection. They can respond to multiple signals by producing several types of cytokines and other inflammatory factors that recruit specific cells to the site of infection. They also have a role in downregulating inflammation and contribute to the resolution of inflammation (Mayadas et al., 2014).

Figure 1: Diagram conveying the different effector anti-microbial functions of neutrophils. (Taken from Rosales, 2018)

Neutrophil Granulation

Neutrophils cause degranulation of pathogens by releasing granules containing antimicrobial factors. The type of granules change depending on the stage that the neutrophils have been produced. Primary granules are found at the initial stage and are called azurophil granules. Secondary granules are found at the next stage and are called specific granules. Tertiary granules are found in the end stage of the neutrophil life cycle and are called gelatinase granules (Wikipedia Contributors, 2020c). The table below outlines the type of granules released by neutrophils and the antimicrobial factors within each of the differing granules.

Granule Type Antimicrobial Factors

Azurophil Granules

Specific Granules

Tertiary Granules

Cathepsin, gelatinase, collagenase

References

Mayadas, T.N., Cullere, X., and Lowell, C.A. (2014) The multifaceted functions of neutrophils. Annu Rev Pathol 9: 181-218.

Rosales, C. (2018) Neutrophil: A Cell with Many Roles in Inflammation or Several Cell Types? Front Physiol 9: 113.

Wikipedia Contributors, W., (2020) Neutrophil.

15th Apr 2020 Sarah Donovan MSc

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