Our defense mechanism makes us attractive

To people and mosquitoes

By Bart Braun

Scientists from Leiden and Wageningen have discovered there might be a connection between the gene for our immune system and how attractive we are to mosquitoes.

If you ever happened to share a bedroom with someone else, you might have noticed the unfair distribution of mosquito bites: one sleeper will wake up covered in red weals while the other’s skin is as clear as a baby’s bottom in a nappy advert. It seems that some people are more appealing to mosquitoes than others. But isn’t that odd? You would think that blood is just blood and what difference could it make to a mozzie? The question is a crucial one when the conversation turns from the painful bites we get from Dutch mosquitoes to malaria. Approximately half the people who ever died, died of malaria and the disease is still killing hundreds of thousands of people every year, causing millions of Euros’ worth of economic damage. That is why, for more than twenty years, scientists in Wageningen have been studying exactly what a mosquito does.

Former Wageningen researcher Bart Knols has written an unrivalled book, Mug [Mosquito], presenting an overview of all the research on mosquitoes from all over the world. For mosquitoes, it scarcely matters how often you wash or whether you chew garlic or swill beer, whatever popular belief says. And turning off all the lights in your bedroom at night won’t have any effect; mosquitoes are not attracted to light but to heat, the carbon dioxide you breathe out and the smell of your sweat.

Of course, sweat itself doesn’t smell and only produces an odour when bacteria start to eat it. Knols made global headlines when he discovered that mosquitoes were devoted to Limburg cheese, which is made with the same strain of bacteria as the one that grows on sweaty feet. Presumably, the first manufacturers of this cheese pressed the curds with their bare feet, accidently adding the bacteria to the recipe. So, your smelly socks don’t smell of cheese – it’s the other way round.

While mosquitoes love the smell of sweat, not everyone’s sweat is the same: age, diet and health all influence the scent. Moreover, our genes have a part in it and that is how Leiden professor of immunogenetics, Frans Claas, became involved.

The mosquito research was a diversion for Claas, as he usually concentrates on the genetics of the immune system, concentrating on the delicate balance of the defence system during pregnancy: all foreign bodies should be repulsed except the baby. This field is also crucial for finding the best donor for a patient, as the recipient’s chances improve tremendously if the donor and recipient resemble each other in immunogenetic terms.

The Human Leukocyte Antigens (HLAs), proteins that are in all sorts of cells in our bodies acting as a sort of security badge, must bear the most similarities. The HLAs present substances that are on the inside of the cell and the body’s security system – the white corpuscles – check whether everything is correct. No badge? Then it must be a bacterium or parasite that should be disposed of. The immune system also kicks in if the substances are HLA proteins but are full of virus substances.

The greater the variety of HLAs you can produce, the more pathogens you can recognise on time. Organ donors and recipients must be as similar as possible as far as HLAs are concerned, but parents must differ as much as possible as that means their genes will be all mixed up and the child will have a large selection of "badges". "The more diverse your genes are, the better your defence mechanism will be", says Claas succinctly.

Consequently, it is no coincidence that HLA genes are closely linked to body odour and this allows you to subconsciously know whether someone’s immune system suits you. In 1995, a Swiss researcher discovered that people prefer the scent of someone whose package of "badges" differs to their own.

Claas and his colleagues discovered that HLA genes also play a role in how attractive you are to mosquitoes. In the journal Infection, Genetics and Evolution, they describe a study among 48 volunteers who were asked to wipe their feet on glass marbles. The mosquitoes definitely preferred the sweat of people with a certain HLA variant.

"We’re not quite sure how it works", Claas continues. "Perhaps the substances connect to aromatic substances in a different way and that accounts for the differences in scent." Alternatively, the differences in defence mechanisms might produce different bacterial flora on the skin that process human sweat in a different way. "At the moment, we don’t which of the two possibilities is correct."

He adds "The next step is find out how it works and what can we do to intervene. But first I want to do a more exhaustive, wider study. It’s not clear whether the effect really is as great as would seem from this research." In any case, he stresses that the results don’t mean that you are safe if don’t happen to have that particular antigen. "Mosquitoes might vary in their preferences too."

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