Q&A: The Body's Response to Injuries and Infections (Innate Immunity)

Humans live in an environment teeming with micro-organisms and could not exist as a species without highly effective mechanisms of host defense. The innate immune system constitutes the first-line barrier, the rapid-response mechanism, to prevent microbial invasion. Its components are inherited from parent to child and directed against molecules expressed only by micro-organisms. These host defense components are evolutionarily ancient, found in all multicellular organisms, and expressed in humans as conserved elements (homologs) shared with other vertebrates and, in some form, with insects and plants.

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In this article:
MCQ: clinical scenario
MCQ: answer
MCQ: explanation

MCQ: clinical scenario

A carpenter gets a splinter of wood embedded in his finger pulp.

What is the first line of defense against this foreign body?

a). Macrophages
b). Neutrophils
c). B-lymphocytes
d). T-Lymphocytes
e). Monocytes

MCQ: answer

The correct answer is B.

MCQ: explanation

Neutrophils, which are also known as polymorphonuclear leukocytes (PMN), represent 50 to 60% of the total circulating leukocytes. They constitute the 'first line of defence' against infectious agents or 'nonself' substances that penetrate the body's physical barriers. Once an inflammatory response is initiated, neutrophils are the first cells to be recruited to sites of infection or injury.

The Neutrophil Granulocyte
Neutrophil granulocytes [also neutrophils or polymorphonuclear leukocytes (PMNs)] are the most abundant (40 to 75 %) type of white blood cells in mammals and form an essential part of the innate immune system. They are formed from stem cells in the bone marrow. They are short lived and highly motile. The average lifespan of non-activated human neutrophils in the circulation is about 5.4 days. Upon activation, they marginate (position themselves adjacent to the blood vessel endothelium), and undergo selectin-dependent capture followed by integrin-dependent adhesion in most cases, after which they migrate into tissues, where they survive for 1–2 days.

Neutrophils may be subdivided into segmented neutrophils (or segs) and banded neutrophils (or bands). Below is a slide of segmented neutrophils. They form part of the polymorphonuclear cell family (PMNs) together with basophils and eosinophils.

Neutrophils with a segmented nuclei surrounded by erythrocytes, the intra-cellular granules are visible in the cytoplasm (Giemsa stained). Credit: Dr Graham Beards / CC BY-SA

The name neutrophil derives from staining characteristics on hematoxylin and eosin (H&E) histological or cytological preparations. Whereas basophilic white blood cells stain dark blue and eosinophilic white blood cells stain bright red, neutrophils stain a neutral pink. Normally, neutrophils contain a nucleus divided into 2–5 lobes.

Neutrophils are a type of phagocyte and are normally found in the blood stream. During the beginning (acute) phase of inflammation, particularly as a result of bacterial infection, environmental exposure, and some cancers, neutrophils are one of the first-responders of inflammatory cells to migrate towards the site of inflammation. They migrate through the blood vessels, then through interstitial tissue, following chemical signals such as Interleukin-8 (IL-8), C5a, fMLP and Leukotriene B4 in a process called chemotaxis. They are the predominant cells in pus, accounting for its whitish/yellowish appearance. Neutrophils are recruited to the site of injury within minutes following trauma and are the hallmark of acute inflammation.

Reference(s)
1). Wikipedia: Neutrophil. Available online: https://en.wikipedia.org/wiki/Neutrophil