Why do mosquito bites itch?
A mosquito bite itches because the body reacts to the saliva the mosquito injects: histamine is released in the skin, and a bump swells and itches, peaking 20 to 30 minutes after the bite (Vander Does, 2022). In many adults, a 2nd bump comes back the next day, peaking between 24 and 36 hours.
Schematic drawing, not to scale. Dotted curves are estimates, in relative level, based on published timing; everyone reacts in their own way, depending on the bites they have already had.
Why do mosquito bites itch?
A bite seen in cross-section, from the 1st minute to the next day. Choose a situation, then tap a label on the drawing to find out what it is.
Sources: bump at its peak in 20 to 30 minutes, 2nd bump between 24 and 36 hours, saliva and histamine, children (Vander Does et al., Front Immunol, 2022); bump within minutes, then the next day (CDC); histamine measured in the skin (Horsmanheimo et al., J Allergy Clin Immunol, 1996); age (Oka and Ohtaki, J Dermatol, 1989); antihistamine (Karppinen et al., Acta Derm Venereol, 2006; Reunala et al., Clin Exp Allergy, 1991). Drawing not to scale; sizes in the scene: estimates.
Keyboard: 1 to 3 situations, Space pause, L slow motion, R restart, ← → previous or next step.
Frequently asked questions about mosquito bites
How long does a mosquito bite last?
The 1st bump, swollen and red, peaks 20 to 30 minutes after the bite and fades within a few hours (Vander Does, 2022). In many people, a 2nd bump, small and hard, appears the next day: it peaks between 24 and 36 hours, then fades over several days (Vander Does, 2022; CDC). In a child who has a large reaction, it can last 3 to 10 days.
Why do some bites not itch?
Because the reaction depends on the bites you have already had. People go through 5 stages: first nothing, then only the next-day bump, then both bumps, then only the immediate bump, and finally, sometimes, nothing at all (Vander Does, 2022). In 27 adults, the share of volunteers with antibodies against the saliva went from 19% to 48% after 10 days of mosquito season (Reunala, 1991). Moving to another country and another mosquito species can start it all over again.
Should you scratch a mosquito bite?
No. Scratching brings relief for a few seconds, but can leave scars and dark spots, and let germs in (Vander Does, 2022). An ice pack for 10 minutes reduces swelling and itching (CDC). An antihistamine taken before the bites reduced itching by 62% at 15 minutes in a study of 30 adults (Karppinen, 2006).
Why do children react more strongly?
Because their body is meeting mosquito saliva for the first time. In 162 people aged 1 to 68, the immediate reaction became more common from early childhood to the teenage years, then rarer in adults (Oka, 1989). Children more often have large reactions, red, warm and swollen, sometimes with fever: skeeter syndrome, which goes away in 3 to 10 days (Vander Does, 2022).
When should a mosquito bite worry you?
It is best to see a doctor if the bite becomes red and warm, with a red streak spreading from the bump: this is a sign of infection (CDC). Trouble breathing or a swelling face is an emergency. After a trip to a region where mosquitoes carry malaria or dengue, a fever needs prompt medical advice. These diseases, along with others spread by vectors, cause more than 700,000 deaths a year worldwide (WHO).
True or false?
Mosquito saliva itself contains histamine.
True. Mosquito saliva contains enough histamine to cause itching, on top of what our own mast cells release (Vander Does, 2022).
A bite makes only one bump.
False. In many people, a 1st bump swells within 20 to 30 minutes, then a 2nd one, small and hard, comes back the next day, peaking between 24 and 36 hours (Vander Does, 2022; CDC).
Adults always react more strongly than children.
False. It is rather the opposite: children more often have large reactions, and the immediate reaction becomes rarer in adults who are often bitten (Oka, 1989; Vander Does, 2022).
This page as text
The same information as the charts, to read without animation: for each chart, the values at the main moments, then the notes with their level of evidence.
Why does it itch in the minutes after?
Because mosquito saliva makes the skin release histamine: the bump swells and itches, peaking around 20 to 30 minutes. Estimated curves, in relative level, in an adult who has often been bitten.
| Time after the bite | Swollen bump (estimate) (dotted) | Itching (estimate) (dotted) | Histamine in the skin (estimate) (dotted) | What happens |
|---|---|---|---|---|
| Bite | 0 | 0 | 5 | The bite: saliva enters the skin |
| 15 min | 88 | 99 | 45 | after the bite: at its peak |
| 30 min | 98 | 91 | 90 | after the bite: at its peak |
| 60 min | 55 | 50 | 60 | after the bite: the bump goes down |
| 90 min | 25 | 25 | 25 | after the bite: the immediate reaction fades |
| 120 min | 10 | 15 | 5 | after the bite: the immediate reaction fades |
Key points
- A bump within minutes (Until 4 min, Shown in humans)
A swollen, reddish bump appears a few minutes after the bite. Source: CDC, « About Mosquito Bites », 2024 - Peaking around 20 to 30 minutes (From 10 to 35 min, Shown in humans)
The round bump, 2 to 10 mm, red around the edge, peaks 20 to 30 minutes after the bite. This is the immediate reaction, caused by IgE antibodies. Source: Vander Does, Labib and Yosipovitch, « Update on mosquito bite reaction: Itch and hypersensitivity, pathophysiology, prevention, and treatment », Front Immunol, 2022 - Histamine in the saliva itself (Throughout, Shown in humans)
Mosquito saliva itself contains enough histamine to cause itching, on top of what the skin's mast cells release. Its D7 proteins are the main allergens, from one mosquito species to another. Source: Vander Does, Labib and Yosipovitch, « Update on mosquito bite reaction: Itch and hypersensitivity, pathophysiology, prevention, and treatment », Front Immunol, 2022 - Histamine measured in the skin (From 25 min, Shown in humans)
In 6 sensitive people, histamine rose in the skin 30 to 45 minutes after the bites, then returned to normal within 2 hours. Source: Horsmanheimo et coll., « Histamine and leukotriene C4 release in cutaneous mosquito-bite reactions », J Allergy Clin Immunol, 1996 - You become sensitized by getting bitten (Throughout, Shown in humans)
In 27 adults, the share of volunteers with antibodies against the saliva went from 19% to 48% after 10 days of the season and 47 bites on average. Source: Reunala et coll., « Cutaneous reactivity to mosquito bites: effect of cetirizine and development of anti-mosquito antibodies », Clin Exp Allergy, 1991 - Estimated curves (Throughout, Hypothesis)
The curves give the timing of the reaction, in relative level. Their height varies a lot from person to person.
Why does the bump come back the next day?
Because a 2nd, slower reaction comes from white blood cells: small, hard, itchy bumps, peaking between 24 and 36 hours. Estimated curves, in relative level.
| Time after the bite | Adult: 1st bump, right away (estimate) (dotted) | Adult: 2nd bump, the next day (estimate) (dotted) | Young child: stronger 2nd bump (estimate) (dotted) | What happens |
|---|---|---|---|---|
| Bite | 0 | 0 | 0 | The bite |
| 12 h | 0 | 30 | 60 | after the bite: the 2nd bump rises |
| 24 h | 0 | 60 | 95 | after the bite: the 2nd bump at its peak |
| 36 h | 0 | 60 | 96 | after the bite: the 2nd bump at its peak |
| 48 h | 0 | 42 | 80 | after the bite: it fades over several days |
| 72 h | 0 | 16 | 45 | after the bite: it fades over several days |
| 96 h | 0 | 5 | 20 | after the bite: it fades over several days |
Key points
- The 1st bump goes away in a few hours (Until 6 h, Shown in humans)
The immediate bump appears within minutes and fades within a few hours. Source: Vander Does, Labib and Yosipovitch, « Update on mosquito bite reaction: Itch and hypersensitivity, pathophysiology, prevention, and treatment », Front Immunol, 2022 - The 2nd bump: 24 to 36 hours (From 12 to 48 h, Shown in humans)
A delayed reaction, driven by lymphocytes, causes small, hard, itchy bumps, peaking between 24 and 36 hours. They fade gradually over several days. Source: Vander Does, Labib and Yosipovitch, « Update on mosquito bite reaction: Itch and hypersensitivity, pathophysiology, prevention, and treatment », Front Immunol, 2022 - A hard, reddish brown bump (From 12 to 36 h, Shown in humans)
About a day after the bite, a hard, reddish brown, itchy bump can appear. Source: CDC, « About Mosquito Bites », 2024 - 5 stages over the bites (Throughout, Shown in humans)
At the very start, a small red spot; then a 2nd bump alone; then both; then the 1st bump alone; and sometimes nothing at all after years. Everyone moves at their own pace: in one study, 6 people out of 10 still had both bumps after 30 years. Source: Vander Does, Labib and Yosipovitch, « Update on mosquito bite reaction: Itch and hypersensitivity, pathophysiology, prevention, and treatment », Front Immunol, 2022 - It changes with age (Throughout, Shown in humans)
In 162 people aged 1 to 68 bitten by the tiger mosquito, the immediate reaction became more common from early childhood to the teenage years, then rarer in adults. Source: Oka and Ohtaki, « Clinical observations of mosquito bite reactions in man: a survey of the relationship between age and bite reaction », J Dermatol, 1989 - Children: large reactions (From 24 h, Shown in humans)
Children more often have large reactions, red, warm and swollen, sometimes with fever: this is skeeter syndrome, which goes away in 3 to 10 days. Source: Vander Does, Labib and Yosipovitch, « Update on mosquito bite reaction: Itch and hypersensitivity, pathophysiology, prevention, and treatment », Front Immunol, 2022 - More than an itch (Throughout, Shown in humans)
Worldwide, diseases spread by mosquitoes and other vectors cause more than 700,000 deaths a year; malaria alone, 249 million cases and more than 608,000 deaths. Source: WHO, « Vector-borne diseases », fact sheet, 2024 - Estimated curves (Throughout, Hypothesis)
No study gives these curves hour by hour: they show the timing of the 2 reactions, in relative level.
What soothes a mosquito bite?
An antihistamine taken in the days before reduces the bump and the itching; heat or cold mostly soothe the itching. Estimated curves: itching, in relative level.
| Time after the bite | Nothing (estimate) (dotted) | Antihistamine taken in the days before (estimate) (dotted) | Heat pen right after (estimate) (dotted) | What happens |
|---|---|---|---|---|
| Bite | 0 | 0 | 0 | The bite: saliva enters the skin |
| 15 min | 99 | 37 | 22 | after the bite: at its peak |
| 30 min | 91 | 35 | 32 | after the bite: at its peak |
| 60 min | 50 | 19 | 26 | after the bite: the bump goes down |
| 90 min | 25 | 10 | 16 | after the bite: the immediate reaction fades |
| 120 min | 15 | 6 | 10 | after the bite: the immediate reaction fades |
Key points
- Antihistamine: 62% less itching (Throughout, Shown in humans)
In 30 sensitive adults, levocetirizine 5 mg, taken in the days before, reduced the bump by 60% and the itching by 62% at 15 minutes, compared with a placebo. At 24 hours, the bump was 71% smaller and the itching 56% weaker, but only 8 people were measured at that point. Source: Karppinen et coll., « Levocetirizine for treatment of immediate and delayed mosquito bite reactions », Acta Derm Venereol, 2006 - The 2nd bump: not always (Throughout, Shown in humans)
In another study, in 27 adults, cetirizine reduced the 1st bump and the itching, but not the delayed reaction. Source: Reunala et coll., « Cutaneous reactivity to mosquito bites: effect of cetirizine and development of anti-mosquito antibodies », Clin Exp Allergy, 1991 - Heat: a study without a placebo (Until 10 min, Shown in humans)
In about 1,750 users of a heat pen, mosquito bite itching fell by 57% in the 1st minute and by 81% after 5 to 10 minutes. The study was not blinded. Source: Metz et coll., « Efficacy of concentrated heat for treatment of insect bites: a real-world study », Acta Derm Venereol, 2023 - Cold: 10 minutes (Until 10 min, Shown in humans)
An ice pack, 10 minutes, reduces swelling and itching. Source: CDC, « About Mosquito Bites », 2024 - Do not scratch (Throughout, Shown in humans)
Scratching can leave scars and dark spots, and let germs in. Source: Vander Does, Labib and Yosipovitch, « Update on mosquito bite reaction: Itch and hypersensitivity, pathophysiology, prevention, and treatment », Front Immunol, 2022 - Estimated curves (Throughout, Hypothesis)
The curves show the direction of the effect, in relative level: no study compares these 3 approaches with each other.
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