What are the symptoms and consequences of the allergic cascade?

When histamine is injected into the skin, a technique used in diagnosing allergies, a reaction that can mimic an allergic reaction occurs. The histamine injection prompts the development of a pale, central swollen area that is caused by fluid leaking out of local blood vessels into the adjacent tissues. This localized reaction is called a "wheal." A red "flare," which sometimes has a warm feeling due to inflammation, surrounds this "wheal." Itching occurs because histamine irritates the nerve endings in the skin.

This early or immediate response peaks at about 15 minutes and symptoms fade within 90 minutes. Sometimes, the immediate effects are followed by a late-phase reaction with symptoms occurring about four to six hours later and lasting up to a day.

Allergens, such as ragweed pollen, react with the tissues lining the inner surfaces (membranes) of the nose and eyes, thereby triggering mast cells to release chemical mediators, including histamine. The chemical mediators cause a leakage of fluid and the production of mucous, causing a runny nose, itching, and sneezing. The late reaction also causes the tissues to swell and the nose to become congested.

In the lungs, exposure to inhaled allergens causes wheezing, shortness of breath, and coughing within seconds or minutes. These symptoms tend to subside after about an hour. However, after about four hours, the late-phase reaction can cause a worsening of shortness of breath, wheezing, and coughing. This phase can last for up to 24 hours. The late-phase reaction involves an influx of a variety of inflammatory cells (eosinophils, neutrophils, lymphocytes, and mast cells) to the affected area, and if repeated inhalations of allergens cause recurrent reactions, these reactions may merge into each other leading to chronic or persistent allergic asthma.

Lastly, allergens can be absorbed into the bloodstream in a systemic reaction and travel to many sites (including the nose, lungs, throat, skin, and digestive tract), causing multiple symptoms that are typical of a severe allergic reaction (anaphylaxis). Blood vessel dilation may occur throughout the body causing a drop in blood pressure and shock. Although rare, this type of anaphylactic reaction can be caused by medications, insect venoms, and foods.