Allergic reactions are sensitivities to substances, called allergens, that are contacted through the skin, inhaled into the lungs, swallowed, or injected.

Allergic reactions are common. The immune response that causes an allergic reaction is similar to the response that causes hay fever. Most reactions happen soon after contact with an allergen.

Many allergic reactions are mild, while others can be severe and life-threatening. They occur more often in people with a family history of allergies.

Substances that don't bother most people (such as venom from bee stings and certain foods, medications, and pollens) can trigger allergic reactions in certain people.

While first-time exposure may only produce a mild reaction, repeated exposures may lead to more serious reactions. Once a person has had an allergic reaction (is sensitized), even a very limited exposure to a very small amount of allergen can trigger a severe reaction.

Allergic reactions can be mild or serious. They can be confined to a small area of the body or may affect the entire body.

Most severe allergic reactions occur within seconds or minutes after exposure to the allergen. However, some reactions can occur after several hours, particularly if the allergen causes a reaction after it has been ingested. In very rare cases, reactions develop after 24 hours.

Anaphylaxis is a sudden and severe allergic reaction that occurs within minutes of exposure. Immediate medical attention is needed for this condition. It can get worse very fast and lead to death within 15 minutes if treatment is not received.

Common allergens include:

• Animal dander
• Bee stings or stings from other insects
• Foods, especially nuts, fish, and shellfish
• Insect bites
• Medications
• Plants
• Pollens