[Tracy]

Tracy asked the Naked Scientists:

When insect or spiders bite us, why does it itch?  They bite us and then go away, what's in it for them that we itch after they're gone?

What do you think?

[JimBob]

The insect injects chemicals, such as numbing agents and anti-coagulants injected by mosquitoes, when they bite. These chemicals cause the human body to produce histamine and the itching of this histamine reaction is part of the bodies immune defense system.

[blakestyger]

You're right - but why the itch?

This may be a superfluous question though, as we tend to think everything has a purpose or reason  - and as we are scientists there is no room for teleology in our deliberations.

[rosalind dna]

The damaged nerves in the skin are trying to "knit" together again so that might be why or your auto-immune system is reacting badly at the sting.

[chris]

It's got nothing to do with nerves knitting back together or autoimmune manifestations I'm afraid.

The itch is effectively an allergic reaction to the proteins injected into the body by the mosquito. When the female mosquito bites she squirts saliva from her proboscis into the skin. The saliva contains anti-clotting and immune-evasion chemicals, which protect the mosquito while she feeds.

However, once the meal is complete, the subsequent effect of these injected proteins is to provoke an immune respose which includes the release of histamine; histamine causes blood vessels to open up (vasodilatation), hence the swelling and redness, and it also activates itch-sensitive nerve endings, hence the insatiable desire to scratch.

The purpose of itch is to alert an individual to a problem somewhere on their body's surface. This can be very useful acutely (i.e. when an irritant that can be removed is present), but it's less beneficial after the cause has gone away.

Chris

[thedoc]

Listen to the answer to this question on our podcast by clicking here

[opus]

Is it a happy accident for the mosquito then that the irritant makes us scratch the bite site, so bringing more blood to the skin surface for the mosquito to come back and feed on again?

[Karen W.]

I hate mosquitos!! They love me and eat me up! I am the worlds worse scratcher.... although I have seen children scratch them till they bleed slightly also.....

Can you tell me why after I have scratched it to death and it has bled...that the itch goes away, but not until that first layer is gone..

 In attempts to not scratch I have resorted to bandages and after bite meds that generally don't work because I find them too late... I have heard ammonia and water solution was basically the same as after bite sticks etc... Is that true?

[Karen W.]

BY THe way Long time no see.. welcome back Opus!

[opus]

Thankyou Karen W- it's nice to be back!