We all possess blood-type markers. These are are chemicals released by each specific blood type. In one of the studies that explored the relation of these markers to susceptibility to mosquito bites, the investigators found persons with Type O blood suffered more mosquito bites due to the odor-emitting markers they emit. This same study also suggested that for any blood type, people who secreted the blood marker of their blood type through their skin were more prone to be targeted by mosquitoes than people who did not secrete those markers. It is argued that 24% in the case of the Type O’s. Other studies claim that about 15% of the population do not emit chemical markers of their blood type through their skin, which indicates that there must be some other factor(s) causing the mosquitoes to chase these people.
Some people possess a skin surface that has high concentrations of steroids or cholesterol, which makes their type of skin more attractive to mosquitoes. This is not to say that mosquitoes are more likely to go after people with higher overall levels of blood cholesterol, but such individuals metabolize cholesterol better, so the byproducts of it accumulate of the surface of their skin.
Mosquitoes also target people who produce excess amounts of certain acids, such as uric acid. These substances are known to stimulate the sense of smell in mosquitoes. Another possible mosquito attractant is CO2 or carbon dioxide.