Such reactions to substances, as diverse as pollen, penicillin and bee venom, vary in severity from the very mild to anaphylactic shock. Thus the seriousness of an allergic condition may vary from, for example, a mild rash to a life threatening state. In principle any substance can cause an allergic reaction. However, allergic reactions are usually in response to ingested, injected or inhaled substances, such as food or chemicals. Other causes of allergy may be dust mites, pollens, moulds, animals or insects.
Some allergic reactions occur as a result of skin contact.
Occupational allergy due to exposure to a particular substance over a period of time may also occur. Diagnosis of allergies may be difficult and time consuming. Diagnosis in children with a suspected food allergy is less difficult than in adults as allergic reactions to a particular food occur very quickly. Where the reaction is quick, skin tests can be helpful. Spontaneous remissions from symptoms may also occur.