Which condition occurs in the first hour after exposure to an allergen?

Immediate allergic reactions occur within minutes to a few hours after exposure to an allergen. They are characterized by a rapid onset of symptoms, which may include hives, swelling, difficulty breathing, or anaphylaxis. This type of reaction is primarily mediated by immunoglobulin E (IgE) and involves the release of histamines and other chemical mediators from mast cells and basophils upon re-exposure to the allergen. The speed and severity of these reactions highlight the body's quick response to perceived threats.

In contrast, delayed allergic reactions typically manifest hours to days after exposure, chronic allergic reactions persist or recur over time, and a severe allergic reaction, while it can be immediate, is more categorized by its intensity rather than the timing of symptom onset. Understanding the timing helps in effective management and response to allergic emergencies in a clinical setting.