How long does a allergic reaction take to occur?

Allergic reactions can happen within seconds or minutes of exposure to an allergen, or they can occur hours or even days later. This difference in timing depends on the type of allergen and the person's sensitivity to it.

IgE-mediated reactions are the most common type of allergic reaction. These reactions typically occur within minutes of exposure to an allergen. Examples of IgE-mediated reactions include allergic rhinitis (hay fever), allergic conjunctivitis (eye allergy), and food allergies.

Non-IgE-mediated reactions, also known as delayed hypersensitivity reactions, typically occur hours or days after exposure to an allergen. Examples of non-IgE-mediated reactions include allergic contact dermatitis (rash from touching an allergen) and poison ivy.

The speed of an allergic reaction also depends on the frequency of exposure to an allergen. With repeated exposure, the body can become more sensitive to an allergen, and reactions can occur more quickly and severely. This is why people who are allergic to certain foods or medications may experience more severe reactions if they are exposed to those substances again after a period of time.