The short answer is: the itch is an allergic to mosquito spit.
简单回答：痒是因为对蚊子唾液过敏。
Mosquitoes use a mechanism involving saliva to get as much blood out of you as quick as they can.
蚊子使用唾液的机制以便尽快尽量多吸你的血。
A mosquito’s mouthparts are made up of multiple tubes.
蚊子的嘴由多个管子组成。
The mosquito uses one tube to slurp up blood, Another is used to pump saliva into your blood vessel.
蚊子用一根管子吸血，用另一根管子将唾液注入你的血管中。
The saliva acts as an anticoagulant—preventing blood from clotting and thus making it easier for the blood to flow without disruption.
唾液起到抗凝剂的作用-防止血液凝结，从而使血液更容易流动而不会受到干扰。
But, as it turns out, humans are allergic to some of the proteins in mosquito saliva.
但是，事实证明，人类对蚊子唾液中的某些蛋白质过敏。
even though the saliva is harmless in and of itself, our immune systems are hypersensitive to it.
即使唾液本身无害，我们的免疫系统也对此过敏。
The body reacts to the saliva by releasing histamine, a biological substance that dilates blood vessels in order to facilitate the arrival of white blood cells, blood plasma proteins, and other immune system tools at the site of injury.
人体通过释放组胺来对唾液做出反应，组胺是一种扩张血管的生物物质，以促进白细胞，血浆蛋白和其他免疫系统工具到达受伤部位。
In the process, however, histamine causes inflammation and itching.
但是，在此过程中，组胺会引起炎症和瘙痒。
And although mosquito saliva is harmless, it can carry dangerous diseases, such as West Nile virus, Zika virus, or malaria.
尽管蚊子唾液无害，但它能携带危险的疾病，例如西尼罗河病毒，寨卡病毒或疟疾。
