The common frustration of why some people are constantly targeted by mosquitoes while others are left untouched suggests these insects possess a highly selective process for choosing a meal. Only female mosquitoes bite, as they require the proteins and compounds in blood to develop their eggs. This necessity has driven the evolution of sophisticated sensory mechanisms allowing them to locate and assess potential hosts with precision. The selection process is not random, involving a complex interplay of gases, visual cues, body heat, and a person’s unique chemical signature that determines individual attractiveness.
How Mosquitoes Locate a Host
A mosquito’s search for a host begins with long-range detection, primarily focused on carbon dioxide (\(\text{CO}_2\)) plumes exhaled during breathing. Specialized receptors on the antennae and maxillary palps allow them to sense changes in \(\text{CO}_2\) concentration from distances of up to 20 to 50 meters away. This exhaled gas acts as a beacon, signaling the presence of a warm-blooded animal and guiding the mosquito toward the source.
Once the mosquito is closer, typically within 5 to 15 meters, it integrates other sensory information to narrow its search. Visual cues become relevant at this mid-range, as mosquitoes are often attracted to objects providing high visual contrast, such as dark clothing or moving targets. Finally, upon closing the distance to less than a meter, the insect switches to thermal detection, using infrared sensors to pinpoint areas of skin with higher temperatures and blood flow.
The Science Behind Blood Type Preference
Research indicates that a person’s ABO blood type can influence their attractiveness to certain mosquito species. Studies have shown that mosquitoes tend to land significantly more often on individuals with Type O blood compared to those with other blood types, such as Type A. In controlled settings, individuals with Type O blood were found to be nearly twice as attractive as those with Type A blood, with Type B individuals falling in between.
This preference is not based on the blood itself, but on a secondary genetic trait known as “secretor status.” Approximately 80 to 85% of people are secretors, meaning they possess a gene that causes them to secrete blood type antigens onto the surface of their skin and into other bodily fluids. Mosquitoes are believed to detect these secreted antigens before biting, effectively determining the host’s blood type through skin odor.
For a Type O secretor, the body releases the H antigen, which acts as the precursor to the A and B antigens in the ABO system. This specific chemical signal, detectable on the skin’s surface, appears particularly appealing to several common mosquito species, making Type O secretors the most frequent target. The ability of mosquitoes to sense these secreted markers adds chemical specificity to their host selection process.
Key Chemical Attractants Released Through Skin
While \(\text{CO}_2\) serves as the initial long-distance lure, a complex blend of volatile organic compounds emitted from the skin seals the mosquito’s decision to land and feed. The human skin is home to a diverse community of bacteria that metabolize compounds in sweat, producing a unique odor profile called the skin volatilome. This microbial activity generates several of the most potent short-range attractants.
Lactic acid, a byproduct of exercise and sweat, is a well-established mosquito attractant. Although weakly attractive on its own, its presence significantly enhances the mosquito’s response to other cues, such as \(\text{CO}_2\). Other metabolites like ammonia, uric acid, and various short-chain carboxylic acids are also released through the skin and sweat, contributing to the overall attractive blend.
The exact composition and concentration of these chemicals vary greatly from person to person, which is a major reason for differences in mosquito attraction. For instance, the combination of lactic acid and ammonia, along with \(\text{CO}_2\), is frequently used in commercial mosquito lures to maximize trap efficiency. The quantity of these specific volatile compounds on an individual’s skin directly determines how appealing their odor is to a female mosquito.
Why Individual Physiology Matters
The amount and combination of attractants a person emits are regulated by underlying physiological and genetic factors. The composition of an individual’s skin microbiota, the community of bacteria residing on the skin, plays a major role in determining their unique odor profile. Individuals highly attractive to mosquitoes often have a higher abundance of bacteria, but a lower diversity of bacterial species on their skin.
Physiological states that increase metabolic rate and body temperature also elevate a person’s attractiveness. Pregnant individuals are often more attractive because they exhale a greater volume of \(\text{CO}_2\) and have a slightly higher average body temperature. Engaging in physical exercise increases the production of \(\text{CO}_2\) and lactic acid, making a recently active person a more likely target. Alcohol consumption is another factor that can increase a person’s risk of being bitten, likely by slightly increasing body temperature and altering the release of volatile compounds.