Anaphylaxis is caused by an extreme immune system reaction. Allergens that trigger anaphylaxis can be introduced to the body in several ways, including:

• Injection. Frequently, anaphylaxis is the result of an injection of the allergen under a person’s skin. This includes:
  
  
• • Bee, hornet, wasp, yellow jacket, fire ant and other insect stings.
    
    
  • Syringe-and-needle injections of some vaccines (particularly those made with horse serum).
    
    
  • Blood transfusions (not really anaphylactic, but anaphylactoid, which does not involve an allergen or IgE).
    
    
  • Allergens injected into a patient during allergy skin testing or allergy shots (immunotherapy).
    
    
  • Medications, including those used in treatments (e.g., antibiotics) and in testing (e.g., contrast dyes). These reactions may be anaphylactic or anaphylactoid.
    
    
• Ingested. The two most common allergies associated with ingesting allergens are drug allergies and food allergies. While people can be allergic to nearly any type of drug, those most commonly associated with anaphylaxis include:
  
  
• • Antibiotics, especially penicillin and related drugs (usually and anaphylactic reaction)
    
    
  • Aspirin and nonsteroidal anti-inflammatory drugs (NSAIDs) such as ibuprofen (usually an anaphylactoid reaction)
    
    
  • Prescription opiate pain medications, such as codeine (usually an anaphylactoid reaction)

Even small bites of certain foods can trigger anaphylaxis in extremely allergic individuals. In very few cases, mere skin contact with the food can cause problems. Foods known to trigger anaphylaxis include:

• • Seafood (including fish and shellfish)
  • Sesame seeds
  • Peanuts
  • Tree nuts (e.g., almond, walnut, hazel, cashew)
  • Eggs (particularly in children)
  • Cow’s milk (particularly in children)
  • Additives, including food coloring and preservatives (usually an anaphylactoid reaction)
    
    
• Inhaled. Though rare, inhalation can trigger anaphylaxis. For example, those with latex allergies could be in danger after inhaling particles from rubber gloves. Healthcare workers are at particular risk because they are exposed to latex more frequently than the general public. Those with urinary tract abnormalities, children with spina bifida (a birth defect in which part of one or more vertebrae fails to fully develop) and people who have had multiple surgical procedures are also at higher risk for latex allergies.
  
  Medications may also cause reactions by inhalation. Parents, patients or pharmacists preparing medications are most susceptible.

Sometimes a combination of factors is necessary to trigger anaphylaxis. For example, some individuals must be exposed to two or more allergens before they react. In other cases, a combination of allergen exposure and subsequent exercise is to blame for an individual’s anaphylaxis.

Physicians are not always able to determine the cause of an anaphylactic reaction. Cases in which a trigger cannot be identified are labeled idiopathic.