1. Why a Person's First Mosquito Bite Might Not Itch
A commentary supervised by a dermatologist states that newborns may not feel itchiness when bitten for the first time because they lack immunity to the components in mosquito saliva [2]. However, this is a general trend, and individual differences are considered significant. If the physical stimulation of a thin needle piercing the skin were the cause of the itch, it would appear even on the first bite. Yet, dermatology textbooks explain that the symptoms are thought to be caused by an allergic reaction to the saliva components injected while the mosquito feeds [1]. The source of the itch lies in the saliva the mosquito introduces. This raises a new question: why does the mosquito inject saliva when it only needs to suck blood?
2. Why Mosquitoes Inject Saliva While Sucking Blood
Only female mosquitoes suck blood. Blood is said to provide the necessary nutrition for making eggs [3]. However, when blood leaves a wound, it tries to clot. If it clots, the mosquito can no longer suck it. Therefore, mosquito saliva is said to contain proteins that prevent blood from clotting and keep it flowing [4]. If the blood keeps flowing, the mosquito can eat without interruption. From this perspective, the itch is not something the mosquito intentionally causes, but rather a side effect of a mechanism designed to make feeding easier. So, which components in the saliva does the body react to, and how?
3. What Causes Mosquito Bite Itch Is Still Not Settled
A medical review paper presents three ideas regarding the itch from mosquito bites [3]. The first idea is that histamine, which is naturally present in mosquito saliva, acts directly on nerves. The second is an allergic reaction where the body creates antibodies to other saliva components, causing mast cells in the skin to release histamine (a type I allergic reaction). The third is an inflammatory reaction that does not depend on antibodies. The paper also notes that saliva contains histamine in amounts sufficient to cause itching [3]. However, this is still at the stage of "three explanations have been proposed," and it is not a situation where one can definitively say "histamine is the only cause." The way the reaction appears also varies. There is an "immediate type," where a small swelling and itch appear right after the bite and subside in about 1 to 2 hours, and a "delayed type," where swelling or itch appears the next day or later and can take 1 to 2 weeks to heal [2]. What determines which type appears?
4. How Reactions to Mosquito Bites Change in Five Stages
A review paper introduces a stage theory where reactions change as bite experiences accumulate [3]. When bitten for the first time, only a small red dot appears (Stage I). As bites continue, first only the delayed type appears (II), then both immediate and delayed types appear (III), then only the immediate type remains (IV), and finally, neither appears (V). A commentary supervised by Japanese dermatologists describes a similar flow: the delayed type is common in early childhood, only the immediate type in middle age, and reactions tend to become weaker in old age [2]. An English-language explanation states that children react more strongly than adults because they have been bitten fewer times [4]. However, when a person moves to each stage, and their age, varies significantly from person to person. It cannot be said that "adults are never itchy" or "elderly people never react." An English-language explanation for general readers also says that reactions are strongest during the first few bites and become milder as the immune system adjusts with repetition. This may not describe exactly the same process as the five-stage theory [4].
5. How Mosquitoes Know When to Stop Sucking Blood
In 2024, a research team from RIKEN and Tokyo Jikei University School of Medicine reported a signal that stops blood-sucking in the yellow fever mosquito [5]. The team reported that fibrinopeptide A, a substance formed when mammalian blood clots, acts as a signal that stops the yellow fever mosquito from feeding. In this study, the mosquito stopped sucking and left the host before it was full. The research team describes this as "eating to eighty percent fullness" [5]. It is important to note that this study focused on the yellow fever mosquito. From this material, it cannot be said that the same mechanism exists in common Japanese mosquitoes, such as the common house mosquito or the Asian tiger mosquito.
6. How to Record the Timing of Your Mosquito Bite Reaction
If you notice a mosquito bite, try noting the time of the bite and the time the itch appeared. Avoid rubbing the area; if you touch it, do so gently with clean hands. The next day, add notes on whether redness, swelling, or itchiness remains. Reactions that appear immediately correspond to the immediate type, while those appearing the next day or later correspond to the delayed type. If you remember previous bites, try recalling how long it took for the itch to start, comparing immediate and delayed reactions. You can consider whether your experience aligns with the view that the number of bites plays a role. For further reading, one original source is the 2022 review "Update on mosquito bite reaction" in Frontiers in Immunology [3]. The chapter on "Insect Bites" in dermatology textbooks is available in Japanese [1].