Pauline Dickinson, of Palmerston North, asks :-

Why are some people severely poisoned by one mosquito bite, while others merely experience a slight reddening which soon disappears?

Rosie Rawlinson, a biochemist at the London School of Hygiene and Tropical Medicine, responded.

Female mosquitoes require blood to produce eggs. To get a blood meal they must use their proboscis, a needle like structure, to penetrate the skin, enter the blood vessel and siphon up some of our blood. For effective blood feeding the blood must not clot in the proboscis, which would cause it to become blocked. It is also advantageous to the mosquito if the host is unaware they’re being bitten as it means they are less likely to stop the mosquito from feeding.

To overcome these obstacles the mosquito injects saliva, via the proboscis, into the blood vessel. The saliva contains; anticoagulant proteins, which stop the blood clotting, molecules that anesthetise the bite area, so we do not realise we have been bitten, and molecules which have anti-inflammatory effects.

The molecules in the mosquito’s saliva are detected by white blood cells, specific cells in the immune system, whose role is to bind to any molecules perceived as foreign to our body (non-self). The white blood cells set off a chain of reactions leading to the immune system eliminating the non-self molecules from the body thereby eliminating the potential threat to the body’s healthy state.

Each person has a unique immune system and therefore will react differently to the detection of non-self molecules in the body. Some immune systems perceive the mosquito saliva as a severe threat to the body with elimination requiring many cells quickly eliminating the non-self molecules. This is done through increased blood flow to the bite site and increased body temperature at the site causing inflammation. Other immune systems recognise the saliva as non-self but decide the immune cells already at the bite site can deal with the threat themselves. These are the people who only have a mild reaction to the mosquito bite.