All of these may be referred to as allergens if they set off allergic reactions. What truly causes an allergic reaction, however, is not the actual peanut, cat or flower, but rather a misguided reaction of the immune system. This explains why various allergens have an adverse effect on some people but not on others. In the human immune system, protective proteins called antibodies or immunoglobulins are specifically targeted against particular foreign substances. An allergic person, however, overproduces a specific type of antibody called immunoglobulin E, or IgE, in response to certain normally harmless foreign elements.
Allergies can develop at any age, even before birth.
A person's risk of developing allergies is related to their parents' allergy history. It is unclear what determines which substances will trigger a reaction in an allergic person. Common allergy conditions include hay fever, asthma, eczema, and hives. Associated symptoms include runny noses, short breath, red and watery eyes to itchy skin, rashes and stomach aches. The most severe reactions can occur when allergens such as insect venom, medications, vaccines and hormones gain direct access to the blood stream.