After allergy antibodies have been formed in a person's body in response to a particular allergen, an allergic reaction can occur when the person comes in contact with that allergen. Depending on the substance, allergens can be inhaled, eaten, injected, or contacted by the skin. When allergy antibodies are activated by an allergen, they cause body cells to release a substance called histamine, a chemical that dilates blood vessels, promotes fluid secretions, and stimulates nerves that cause muscles to spasm. These reactions create various allergy symptoms.
In a person with hay fever, for example, pollen allergens cause a number of symptoms. When pollen is breathed in through the nose, the release of histamine in the nasal passages causes violent, repetitive sneezing, release of watery fluids, and itching. Sometimes pollen triggers tissue swelling, which can cause blocked nasal passages, with consequent loss of smell and taste. When pollen allergens affect the eyes, they become itchy, red, and watery. Allergens that affect the lungs cause secretion of mucus and inflammation, swelling, and narrowing of the airways, resulting in asthma. Symptoms include spasms of the airways and sudden difficulty in breathing.
Allergic reactions can also be triggered throughout the entire body, rather than in one specific location. Called allergic or anaphylactic shock, this response occurs when many cells throughout the body react simultaneously to an allergen, such as bee sting venom. The person may experience hives or welts on the skin, itching all over the body, asthmatic spasms in the lungs, or a sudden drop in blood pressure (see Shock). An additional danger is the possibility of swelling in the throat, tongue, and larynx (voice box), which can close the upper airways and cause fatal choking.
Food allergies are fairly common, but they are poorly understood. When foods are digested and the nutrients are absorbed in the intestine, substances in the food probably stimulate the allergic response. The reaction can occur in the intestine itself, resulting in cramps or diarrhea; in the skin in the form of eczema; or all over the body, also causing allergic shock. Food allergy, which is an immune response, is often confused with food intolerance, which may cause similar symptoms of gastrointestinal discomfort. Food intolerance, however, is caused by many factors other than an allergen; a person may lack an enzyme to digest a particular food, for instance. The resulting symptoms of food intolerance are not triggered by an immune response.
Researchers have identified a definite genetic predisposition to allergies. For example, if one parent has allergies, there is an increased risk that some of the children will also have allergies, although the children may not be sensitive to the same allergens as the parent. If both parents have allergies, the risk that the children will develop allergies is even greater. The most typical time for allergies to develop is in young adulthood, although allergies can develop in a person of almost any age, even within a few months after birth. Allergies in infants are most commonly associated with foods and viral respiratory infections. For reasons that are not clearly understood, children with allergies tend to outgrow them. The child's body somehow readjusts its response to allergens, even those that cause severe reactions, such as food, drugs (especially penicillin), and stinging insects.
Patients are sometimes surprised by the abrupt onset of allergies in adult life. This can occur when the combination of a person's genetic makeup and a longstanding but unnoticed reactivity to an allergen finally culminates in a detectable disease. This so-called allergic threshold is crossed when allergens finally produce enough reaction in the body to cause detectable symptoms. Several as yet unknown genes may be involved in this process.
Many people think that emotions, such as stress, cause allergies, but most physicians believe this is incorrect. In fact, the opposite may be true: People with allergies live with symptoms that can produce serious emotional upset. For example, a person with asthma may have fears about losing the ability to breathe, or possibly choking to death. Allergies, including asthma, are caused by biological factors, although emotions may aggravate an allergic reaction.
