The experience of being relentlessly bitten by mosquitoes while others remain untouched often leads to the question of why certain individuals are preferred targets. Female mosquitoes require a blood meal for egg production, and they do not choose their hosts randomly. They employ a sophisticated array of sensory cues to locate the most suitable source. While the notion that a person’s blood type might play a role seems like folklore, scientific research confirms this is one factor influencing a mosquito’s decision.
The Primary Target
Controlled studies have consistently demonstrated a preference for Type O blood. Individuals possessing this blood type are nearly twice as likely to attract mosquitoes compared to those with Type A blood. Type B blood generally falls in an intermediate range of attractiveness. This preference is not due to the blood itself, but rather the chemical signals an individual emits that indicate the blood type to the mosquito before it even bites.
This detection mechanism relies on a genetic trait known as “secretor” status. Approximately 80 to 85% of the human population are secretors, meaning they possess a gene that allows them to secrete water-soluble antigens corresponding to their ABO blood type onto their skin. Mosquitoes utilize specialized chemoreceptors to detect these antigens on the skin’s surface.
For a Type O secretor, the body releases the H antigen, which is the precursor molecule to the A and B antigens. This specific chemical signature appears to be highly appealing to various mosquito species. By contrast, Type A secretors release A antigens, which are the least attractive, suggesting the insects actively avoid the chemical compounds associated with that blood group.
Beyond Blood Type
While blood group is a factor, the primary long-range locator for mosquitoes is the carbon dioxide (\(\text{CO}_2\)) exhaled in a host’s breath. Mosquitoes possess a highly sensitive organ called the maxillary palp, which allows them to detect concentration plumes of \(\text{CO}_2\) from as far as 100 feet away. Individuals who exhale more of this gas, such as larger people or pregnant women, create a larger, more distinct plume, making them easier to locate.
As the mosquito closes the distance, it begins to focus on a complex “chemical cocktail” of volatile organic compounds (VOCs) emanating from the skin. The skin’s resident microbiota, a diverse community of bacteria, plays a major role by metabolizing compounds in sweat, such as proteins and fatty acids, into these odorous VOCs. This unique blend of chemicals, which includes lactic acid, ammonia, octenol, and 2-ketoglutaric acid, gives each person a distinct body odor that mosquitoes find appealing.
Lactic acid, in particular, is a potent attractant that increases with physical exertion, making someone who has recently exercised significantly more appealing. The specific composition and relative abundance of the skin bacteria vary widely between individuals. This variation in the microbial profile directly correlates with the amount and type of mosquito-attracting VOCs produced on the skin, explaining why some people are consistently more attractive than others.
The Biting Mechanism
Female mosquitoes seek out a host because they require a blood meal to develop eggs. Male mosquitoes feed exclusively on nectar and plant juices. Once a female mosquito detects the long-range \(\text{CO}_2\) plume, it transitions to medium-range visual and short-range thermal cues to pinpoint the exact location of the host.
Visual cues, specifically dark contrast, become important once the mosquito is within about 30 feet of the target. Mosquitoes orient strongly toward dark-colored objects, which provide a clear visual contrast against the background. This visual attraction guides the insect toward the host, bridging the gap between the distant detection of \(\text{CO}_2\) and the final close-range assessment.
In the final moments before landing, the insect uses thermoreceptors on its antennae to sense the host’s heat, typically around \(34^\circ \text{C}\) for human skin. This thermal detection, integrated with the strong olfactory signals from the skin’s VOCs, confirms the presence of a warm-blooded host. This multi-sensory integration triggers the final landing and biting attempt, confirming the target is a viable blood source for the successful maturation of its eggs.