These substances can trigger mosquitoes' sense of smell, luring them to land on unsuspecting victims.

Allergy testing involves having a skin or blood test to find out what substance, or allergen, may trigger an allergic response in a person.

Those areas that become red and itchy indicate which substances trigger a defensive (allergic) response by your immune system.

If you have allergies, the substances to which you are allergic can trigger symptoms.

Other substances trigger the body's thirst response - positively and/or negatively.

For years researchers have believed that the body releases bradykinin when there is an injury, and the substance triggers pain.

It is unclear what determines which substances will trigger a reaction in an allergic person.

However, once a substance has triggered an allergic reaction, it usually continues to affect the person.

These substances, or allergens, trigger your body to release immunoglobulin E, which is an antibody.

Errors sometimes occur, in part because an innocuous substance may trigger a positive.