An emergency is an unforeseen condition, or set of circumstances, that requires prompt action. A true medical emergency is usually one in which the patient’s life is threatened and the patient needs im­mediate medical" attention. This definition implies that the medical condition is of very recent onset or that it is perhaps part of a chronic illness that has suddenly become more severe. Because it is sometimes difficult for either the patient or the physician to know whether the illness is immediately life-threatening, any similar con­dition that resulted in death should be considered life-threatening. This chapter deals mainly with such emergencies, as well as with serious but nonfatal, acute conditions. Allergic emergencies are those associated with the classic allergy diseases asthma, hay fever, hives, and eczema, as well as allergic reactions to foreign matter, whether the matter is ingested, injected, inhaled, or absorbed by the skin. Although allergic diseases usually involve an immunological mechanism, some conditions closely re­semble an allergic condition in which immunological causes may not have been demonstrated. These will also be discussed when appro­priate.

Foreign substances are those that are not a natural, integral part of the body, that cause allergic reactions resulting in emergencies, or that produce fairly severe reactions. They are generally not danger­ous —that is, they are not poisons, pathogenic organisms (such as viruses or bacteria), or radiation. These foreign, generally innocuous, substances are called, collectively, allergens. They may be inhaled, ingested, injected, or absorbed by the skin. They usually contain proteins that are medium-sized in molecular weight. Almost in- variably, prior contact with either the identical agent or a closely related agent must occur before the allergic reaction is triggered. Allergens that cause allergic reactions may be classified accord­ing to their sources, and are found in Table 3.1. Some of the drugs listed—for example, aspirin and the dyes used in x-ray studies—are not, strictly speaking, allergens, since a true immunological mecha­nism has not been found. The drugs are included both for traditional reasons and because they so closely reproduce reactions to true allergens.