One summer evening, you're blowing the air conditioner when you hear a familiar hum in your ears. You immediately look around alertly for the source of the sound – and find an arrogant mosquito lying on your calf, motionless.
You have quick eyes and quick hands, and take advantage of the fact that the mosquito is not the slightest bit unprepared.
So the question arises: is it still sucking blood at the last moment before it dies, so is the needle in its mouth broken in your skin, or is it left intact on the mosquito?
Mosquitoes use the mouth needle for sucking blood,
It has a delicate construction
To figure this out, let's first take a look at how mosquitoes suck blood.
Every mosquito that bites you is a female mosquito that needs to suck blood to lay eggs [1]. From the time they target you until they are full, there are three steps: penetrating, probing, and eating[2].
The "needle" that the mosquito uses to suck blood is actually not "one", but "a bundle" of needles - a total of six needles. The scientific name for the biology of this structure is "bundle" [2], and it is also called "mouth needle" in mosquitoes, and each needle is actually a "lip tube", and the part where it sucks blood is composed of this bundle of needles and the lower lip that wraps around them [3], and the whole is called the mouthpart [1].
The needle is typically 2 mm long and about 30 microns in diameter [4], and when piercing a person's skin, it is usually inserted half of its length, i.e., an average of 1 mm deep into the skin [5].
In the bundle, the thickest and longest hollow tube is called the "upper lip", which is responsible for puncturing blood vessels and sucking blood, and on either side of it are two sharp jaws and two upper jaws with fine serrations, which are responsible for cutting through your skin and allowing mosquitoes to better support your body [6]. There is also a "throat canal" that mosquitoes use to spit on you and disable your clotting mechanism [1].
The picture shows a schematic diagram of the anatomy of a mosquito mouth needle / [1]
Yes, every time a mosquito sucks your blood, it leaves its saliva.
The whole process of feeding takes several minutes for a mosquito to feed [1], and the mosquito's movement slows down as the blood in its body increases [7].
Mosquitoes are made of chitin[4], and their mouths are not so hard that even if they penetrate the skin in a quasi-static manner, they will inevitably bend and even break[2][8] – not to mention the power of gnashing their teeth when they see mosquitoes. Therefore, there is a real possibility that the needle of the mosquito will be broken when it is swatted.
The mosquito's mouth needle snapped in my skin
What to do
In fact, even if the mosquito does leave its unique legacy needle in your skin after death, there is no need to worry, because it is a small matter for your skin's immune system.
Human skin, as an important barrier for life-saving [9], has immune cells in both the epidermis and dermis.
The mosquito's mouth needle pierces the skin, even if it does not break inside, it constitutes a mechanical injury. As a result, after the human skin is injured, it will mobilize various cells to work together to heal the wound. The entire repair process is divided into coagulation and inflammation, proliferative, and remodeling phases [10].
After the wound is formed, platelets and neutrophils are the first to "kick in" to stop bleeding and engulf microorganisms. Monocytes then "arrive on the battlefield" after 5 to 6 hours, and these monocytes differentiate into macrophages and become the "permanent workhorse" [11].
As the name suggests, the most prominent role of macrophages is phagocytosis, which surrounds and inhales those impurities, and then mixes and breaks them down through enzymes inside them [12]. One part of it (type M1) is mainly responsible for phagocytosis and exerts a pro-inflammatory effect, while the other part (type M2) promotes healing, tissue growth, and exerts anti-inflammatory effects in a later process [11].
Schematic diagram of the process of macrophage phagocytosis of virus / [12]
In addition to the above-mentioned "wounds", the mosquito breaks the needle, leaving a foreign body. The body's specialized "foreign body response" (FBR) process also requires the involvement of macrophages [13].
The "foreign body reaction" begins with the concentrated adhesion of macrophages to the foreign body. The macrophages then change their structure to create a separate space near the foreign object. If foreign bodies are "hard to bite", macrophages will also secrete a factor to break them down. As a result of the relentless efforts of macrophages, the foreign body begins to crack and decompose from the surface, and is eventually swallowed by the macrophages [14].
If the mouthparts left by the mosquito are small enough, then that's the end of the problem.
When the mosquito starts sucking blood, its mouth needle first causes a wound, and if its mouth needle remains in the skin, it will cause a "foreign body reaction"
If, unfortunately, the mosquito leaves its complete mouthparts in your skin, and this sting is too much of a task for macrophages, then the aforementioned foreign body reaction process will stretch the front line and go through a process of "fibrosis".
At this point, the M2-type healing macrophages mentioned above come into play, not only summoning "T lymphocytes" and "fibroblasts" to gather on the surface of the foreign body, but also activating these cells to become a kind of "myofibroblasts" that we have scabbed and scarred thanks to. At the same time, macrophages themselves fuse with each other on the surface of the foreign body to form multinucleated foreign body giant cells, which in turn form a "Hulk"—the formation of such giant cells that in turn forms part of the protein layer on the surface of the foreign body [14][15].
At this point, macrophages and other stroma form a completely new "cyst", which belongs to the "proliferation" phase of wound healing [9].
This type of cyst caused by foreign body invasion is medically called "foreign body granuloma" [16], but judging by the diameter of the mosquito's mouthparts (30 microns) and shape (a smooth spike) [17][18], the probability of such granulomas is extremely small, and even if it does occur, it will basically disappear with the lack of metabolism and blood supply to the skin itself [19], or be eaten by the body's own enzymes [9]. The recovery time and the size of the foreign body are directly related, i.e., the degree of injury.
The figure shows the microstructure of mixed cell granulomas composed of lymphocytes, eosinophils, plasma cells and histiocytes visible in the dermis / [18]
What your body is really afraid of,
Not their mouthparts
In fact, the most terrible thing for your body is never the physical invasion of the superficial layers of the skin, but the germs and anticoagulant factors in mosquito saliva.
In a study on "changes in the body's skin immune response after mosquito bites," researchers found a large number of immune system changes that were much higher than those of ordinary wound and foreign body responses, even excluding minor physical effects on mosquito mouthparts.
Mosquitoes even use human cells to wreak havoc on the human body: the white blood cells that kill the virus themselves become vectors for the transmission of the virus; Arboviruses, on the other hand, spread to the lymph nodes as part of the immune response [20]......
These questions really make the body like a great enemy.
So when you are itched by mosquito bags and upset, in fact, your body cells have already "carried the weight" for us too much behind these bags.
They're really, I'm crying to death.
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[3] Ma Guojun & Wu Chengwei. (2012). Research progress on microneedle design and penetration process mechanics. Progress in Mechanics (03), 314-331.
[4] Swaminathan, V. S. (2006). Mechanics of a mosquito bite.
[5] Kong, X. Q., & Wu, C. W. (2009). Measurement and prediction of insertion force for the mosquito fascicle penetrating into human skin. Journal of Bionic Engineering, 6(2), 143-152.
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[7] MacDougall, C. (2005). Effect of blood meal size on mosquito response to disturbance while blood feeding on a simulated host.
[8] Wu Chengwei.news.sciencenet. (2011) Artificial microneedles: Inspiration from mosquito mouth needles
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[10] Zheng Z, Zhang C, Lin H, Zeng S, Qin X, Cao W, Chen H. [Wound-healing acceleration of mice skin by Sipunculus nudus extract and its mechanism]. Sheng Wu Yi Xue Gong Cheng Xue Za Zhi. 2020 Jun 25; 37(3):460-468. Chinese.
[11] Larouche, J., Sheoran, S., Maruyama, K., & Martino, M. M. (2018). Immune Regulation of Skin Wound Healing: Mechanisms and Novel Therapeutic Targets. Advances in wound care, 7(7), 209–231.
[12] Askabiologist.asu.edu. (2011). Macrophages.Arizona State University
[13] Chandorkar, Y., K, R., & Basu, B. (2018). The foreign body response demystified. ACS Biomaterials Science & Engineering, 5(1), 19-44.
[14] Carnicer-Lombarte, A., Chen, S. T., Malliaras, G. G., & Barone, D. G. (2021). Foreign body reaction to implanted biomaterials and its impact in nerve neuroprosthetics. Frontiers in Bioengineering and Biotechnology, 9, 622524.
[15]. Joel Winders.Dermnetnz. (2017). Foreign body granuloma
[16] Lee, J. M., & Kim, Y. J. (2015). Foreign body granulomas after the use of dermal fillers: pathophysiology, clinical appearance, histologic features, and treatment. Archives of plastic surgery, 42(2), 232–239.
[17] Madden, L. R., Mortisen, D. J., Sussman, E. M., Dupras, S. K., Fugate, J. A., Cuy, J. L., Hauch, K. D., Laflamme, M. A., Murry, C. E., & Ratner, B. D. (2010). Proangiogenic scaffolds as functional templates for cardiac tissue engineering. Proceedings of the National Academy of Sciences of the United States of America, 107(34), 15211–15216.
[18] Hirota, K., Kurosawa, Y., Goto, K., Adachi, K., Yoshida, Y., & Yamamoto, O. (2015). Tick Bite Granuloma: Recommendations for Surgical Treatment. Yonago acta medica, 58(1), 51–52.
[19] Cleveland Clinic. (2021). Epidermis Layers, Function, Structure.
[20] Guerrero, D., Vo, H. T. M., Lon, C., Bohl, J. A., Nhik, S., Chea, S., Man, S., Sreng, S., Pacheco, A. R., Ly, S., Sath, R., Lay, S., Missé, D., Huy, R., Leang, R., Kry, H., Valenzuela, J. G., Oliveira, F., Cantaert, T., & Manning, J. E. (2022). Evaluation of cutaneous immune response in a controlled human in vivo model of mosquito bites. Nature communications, 13(1), 7036.
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Editors: Pan Qian, Yang Min (intern)
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