Allergies are immune reactions in the absence of pathogens. Minor irritants, such as ragweed and cat dander, can trigger the body's immune system to overreact with sneezing, watery eyes and a runny nose. According to WebMD, the substances in the environment that trigger such attacks are generally harmless.
The root cause of a person's allergies depends on a number of factors. A tendency to develop allergies can be inherited from an individual's parents though, as WebMD explains, not toward any single allergen. If one parent has allergies, each child has a 33 percent chance of becoming allergic to one or more allergens. If both parents have allergies, the incidence jumps to 70 percent.
The particular allergens that trigger attacks vary among sufferers, notes WebMD, though certain substances, such as pollen, are more common allergens than others. To develop an allergy to a specific allergen, the person first must be exposed to the substance in the environment, and the degree to which the allergy develops sometimes depends on the amount of the substance and the duration of the exposure. From the time an allergy to a given substance develops, an allergic reaction is likely to occur with every subsequent exposure though medication can be used to control the symptoms.