Judith Swann of Dunedin asks :-

Sandfly, and mosquito, bites itch and itch heaps! But why? And by that I mean what is the evolutionary advantage to the sandfly, or mosquito, in making it's victim suffer so much?

Alexander McLellan, of the University of Otago's Department of Microbiology and Immunology, responded.

The itch is caused by an immune response to the proteins present in the insect saliva. Insect saliva contains proteins that increase blood flow to the bite site (vasodilators) or prevent clotting. Interestingly, we do not itch the first time we are bitten, but develop an immune response to the salivary proteins following one or more bites. The itch response that occurs immediately upon being bitten is a ‘hypersensitivity’ reaction - much like an allergy.

Salivary proteins act together with our own antibodies to trigger mast cells in the skin to release histamine. This causes the intense itch reaction and is why antihistamine medications can provide effective relief. You may have noticed that this itch subsides after a few hours, only to be replaced by a swelling and intense itch that appears 1-2 days later. This is caused by white blood cells (called T cells) migrating into the bite site and causing additional inflammation in response to residual insect salivary proteins. Thus, hypersensitivity immune reactions to insect saliva have an early and a late phase that are mediated by different cells and molecules.

We think that the itch response is an advantage to the host, but a disadvantage to the insect, since it provokes a grooming response in animals which disrupts the feeding by dislodging the insect. Apart from stealing a little blood, insect bites may also transmit parasites. There is some evidence that immune responses to the bite proteins also help to control the entry of the parasites. So, next time you experience that intense itch upon being bitten, bear in mind that this itch response has evolved to provoke the behavioural response useful in dislodging insects. In addition, the itch is a side effect of an immune response that could protect us against parasitic passengers hitching a ride in the bite saliva.