It seems strange, then, that allergic reactions to corticosteroids actually occur—especially since these medications are used to treat allergic reactions. While severe allergic reactions to corticosteroids are extremely rare, they do in fact occur. Most allergic reactions to corticosteroids are less severe, however, and result from the topical formulations—occurring in up to 6% of people. Allergic reactions to oral or injected formulations are rarer, occurring in less than 1% of people. Causes of allergic reactions to corticosteroids may be due to IgE antibodies , or as a result of delayed-type hypersensitivity reactions caused by T-cells (a type of white blood cell).
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 .