Mosquito bites itch because your immune system reacts to the insect's saliva.

When a female mosquito pierces the skin using her proboscis, she injects saliva containing a complex cocktail of over 100 different proteins. These proteins act as anticoagulants to keep the blood flowing and anesthetics to prevent the host from feeling the initial puncture. The human immune system quickly recognizes these foreign proteins as antigens and triggers a localized allergic response.The primary chemical released during this response is histamine, which is produced by mast cells located in the connective tissue. Histamine increases the permeability of nearby capillaries, allowing white blood cells and proteins to reach the affected area. This influx of fluid causes the characteristic swelling known as a wheal, while the dilation of blood vessels creates the surrounding redness, or flare.The itching sensation occurs because histamine also stimulates sensory nerve endings in the skin. Research published by the National Institutes of Health (NIH) indicates that specific proteins like 'Aegyptin' in Aedes aegypti mosquitoes are major triggers for this reaction. Interestingly, repeated exposure can lead to desensitization, which is why some adults react less severely to bites than children do.While the itch is annoying, it serves as an evolutionary warning signal that the skin barrier has been breached. Scientists at institutions like Johns Hopkins University continue to study these salivary proteins to better understand how mosquitoes transmit diseases like Malaria and Zika. Understanding these biochemical interactions is crucial for developing new repellents and vaccines.