Anaphylaxis is a severe allergic reaction, which can cause shock, low blood pressure, and occasionally death. Food allergies, including allergy to peanuts and tree nuts, are said to account for the majority of fatal or near fatal anaphylactic reactions in the . Care is taken especially when chemotherapy medications are known to be common allergic reaction producers, to pre-medicate to prevent or lessen reaction. If a drug has caused a severe allergic reaction in the past it will unlikely be used in the future. It is important to report any allergies or allergic reactions to drugs taken in the past to your health care professional.
Gutworms and similar parasites are present in untreated drinking water in developing countries, and were present in the water of developed countries until the routine chlorination and purification of drinking water supplies.  Recent research has shown that some common parasites , such as intestinal worms (., hookworms ), secrete chemicals into the gut wall (and, hence, the bloodstream) that suppress the immune system and prevent the body from attacking the parasite.  This gives rise to a new slant on the hygiene hypothesis theory—that co-evolution of humans and parasites has led to an immune system that functions correctly only in the presence of the parasites. Without them, the immune system becomes unbalanced and oversensitive.  In particular, research suggests that allergies may coincide with the delayed establishment of gut flora in infants .  However, the research to support this theory is conflicting, with some studies performed in China and Ethiopia showing an increase in allergy in people infected with intestinal worms.  Clinical trials have been initiated to test the effectiveness of certain worms in treating some allergies.  It may be that the term 'parasite' could turn out to be inappropriate, and in fact a hitherto unsuspected symbiosis is at work.  For more information on this topic, see Helminthic therapy .
Successful immediate treatment requires prompt recognition by the attending anaesthetist , or in the US, the attending anesthesiologist or nurse anesthetist. Anaesthetists are trained to recognise if an allergic reaction is occurring. The identification of a complication is made by the recognition of issues such as low blood pressure, hives, wheezing, rash, swelling around the eyes or in the mouth and throat and other breathing difficulties.  Adrenaline (epinephrine) remains the mainstay of treatment, with corticosteroids and antihistamines providing limited benefit in the acute situation.