Читать книгу Emergency Medical Services - Группа авторов - Страница 457

Allergic reactions are hypersensitivity reactions resulting from the exposure to an allergen [1]. In milder forms they may result in localized edema and pruritus. Systemic reactions can also be mild, resulting in a more widespread rash that can be pruritic. In their most severe form, allergic reactions progress to anaphylaxis with multisystem and potentially life‐threatening manifestations that include respiratory failure, circulatory collapse, and shock.

There are four types of hypersensitivity reactions (Box 21.1). Type I accounts for most cases of anaphylaxis. Type II reactions are typically seen in the setting of blood transfusions, drug reactions, and cases of idiopathic thrombocytopenic purpura. Type III reactions are responsible for serum sickness, reactions to tetanus toxoid, and poststreptococcal glomerulonephritis. Type IV reactions are T‐cell–mediated and delayed hypersensitivity responses that do not cause anaphylaxis.

Urticaria, or hives, is an often‐encountered symptom and physical sign of an acute allergic reaction. Although the potential etiologies of urticaria are numerous, the temporal link to a likely allergen can often be made upon consideration of recent exposures. For example, the patient might have recently started a new medication, been stung by an insect, or eaten a certain food. Urticaria, itself, is not particularly concerning. However, its potential as an indicator of a reaction in the evolution of systemic effects toward anaphylaxis should not go unrecognized.