Only female mosquitoes bite humans because they require the proteins and nutrients in blood to develop and mature their eggs. Male mosquitoes feed only on nectar and plant juices for energy. Mosquitoes locate hosts using a complex, multi-layered process of biological signals. This intricate search begins with distant chemical cues and ends with close-range physical signals, determining the most attractive target.
The Primary Attractant
The primary long-range signal mosquitoes use to locate a potential host is the plume of Carbon Dioxide (CO2) exhaled with every breath. Mosquitoes possess highly sensitive receptors on their antennae, allowing them to detect a CO2 concentration gradient in the air. This enables them to track a human host from a distance of up to 50 meters away.
The size of the CO2 plume relates directly to a person’s metabolic rate and breathing volume. Individuals who are larger, exercising, or pregnant naturally exhale greater volumes of CO2, creating a stronger, more easily traceable signal.
Once a mosquito follows the CO2 trail to within a few meters, other sensory inputs take over for final target acquisition. These close-range cues include visual signals, such as the contrast of a person’s silhouette, and thermal signatures. Mosquitoes use their visual system to spot dark clothing more easily, and they use heat sensors to detect body warmth, guiding them to an ideal landing site.
Blood Type and Secretor Status
Mosquitoes show a distinct preference for individuals with Type O blood compared to other ABO blood groups. In controlled studies, mosquitoes landed on people with Type O blood nearly twice as often as those with Type A blood, with Type B individuals falling in the middle. This preference is due to chemical signals a person emits through their skin, not the blood itself.
Approximately 80% of people are classified as “secretors,” meaning they secrete their specific blood type antigens onto the skin surface. These antigens act as odorless markers that mosquitoes can detect before they bite. Secretors with Type O blood emit the H-antigen, which is a precursor molecule for the A and B antigens found in other blood types.
Mosquitoes are statistically more attracted to secretors than non-secretors, regardless of the specific blood type. Among secretors, those with Type O blood are the most attractive because of the specific H-antigen they secrete. This mechanism explains why a person’s genetic makeup plays a significant part in their overall attractiveness.
Metabolic Byproducts and Skin Chemistry
Beyond CO2 and blood antigens, the host’s unique body odor serves as a powerful short-range attractant, composed of various volatile organic compounds (VOCs). A significant component is Lactic Acid, released through sweat, especially after physical activity. Lactic Acid acts as a key synergist, greatly enhancing the attractiveness of other chemical compounds sensed by the mosquito.
Ammonia and other nitrogenous compounds in sweat also contribute to the host-seeking signal. The crucial element in generating appealing odors is the individual’s skin microbiome, the community of bacteria on the skin’s surface. These bacteria metabolize non-smelling compounds in sweat into a bouquet of VOCs, such as carboxylic acids, that are highly attractive to mosquitoes.
Genetic factors influence the composition of these chemical outputs and the type of bacteria that thrive on the skin. Individuals attractive to mosquitoes often harbor a high abundance of a few specific bacterial species, leading to a less diverse skin microbiome. This bacterial imbalance results in a unique chemical signature that makes the person a preferred target.