Ah, Science.

I am one of those people who seems to be slaughtered by mosquitoes when they are out, while people sitting right next to me go largely unscathed. Now, it seems the reason might just be my blood type of all things.
:

A 1972 study in Nature found that mosquitoes were more likely to bite people with type O blood than other potential victims, while people with type A got the fewest bites. ... Conclusion: to quote a favorite line from the Straight Dope Message Board, if you're a type O secretor, to a mosquito you look like caramel-covered crack.

Other studies have cast doubt on this phenomenon, claiming that factors such as sweatiness are more likely to influence victim selection. But a Japanese study from 2004 seems convincing. Researchers exposed 64 volunteers to a swarm of hungry female mosquitoes, each of which had had its proboscis — the part it bites with — amputated. (One can imagine the outraged e-mail this surely elicited from PETA.) Since the mosquitoes couldn't bite (and thus couldn’t drink their fill), the researchers were able to compare how often they landed on the skin of different volunteers. They found type O secretors were twice as attractive to mosquitoes as type A secretors.

How do mosquitoes know about chemicals on your skin? Smell, probably.

So, the good news is that there's apparently a reason mosquitoes attack me, and it's not out of vengeance, spite or malice. The bad news is that I can't do anything about it. Loverly.