The overall result of the type I reaction is an acute inflammation marked by local seepage of fluid from and dilation of the blood vessels, followed by ingress of granulocytes into the tissues. This inflammatory reaction can be a useful local protective mechanism. If, however, it is triggered by an otherwise innocuous antigen entering the eyes and nose, it results in swelling and redness of the linings of the eyelids and nasal passages, secretion of tears and mucus, and sneezing—the typical symptoms of hay fever. If the antigen penetrates the lungs, not only do the linings of the bronchial tubes become swollen and secrete mucus, but the muscle in their walls contracts and the tubes are narrowed, making breathing particularly difficult. These are the symptoms of acute asthma. If the antigen is injected beneath the skin—for example, by the sting of an insect or in the course of some medical procedure—the local reaction may be extensive. Called a wheal-and-flare reaction, it includes swelling, produced by the release of serum into the tissues (wheal), and redness of the skin, resulting from the dilation of blood vessels (flare). If the injected antigen enters the bloodstream and interacts with basophils in the blood as well as with mast cells deep within the tissues, the release of active agents can cause hives, characterized by severe itching. If the antigen enters through the gut, the consequences can include painful intestinal spasms and vomiting. Local reaction with mast cells increases the permeability of the mucosa of the gut, and in many cases the antigen enters the bloodstream and also produces hives. Regardless of whether the allergen is injected or ingested, if it ends up in the bloodstream, it can induce anaphylaxis, a syndrome that in its most severe form is characterized by a profound and prolonged drop in blood pressure accompanied by difficulty in breathing. Death can occur within minutes unless an injection of epinephrine is administered immediately. This type of severe allergic reaction can occur in response to foods, drugs such as penicillin, and insect venom.

Another feature of type I hypersensitivity reactions is that, once the immediate local reaction to the allergen has taken its course, there may occur an influx of more granulocytes, lymphocytes, and macrophages at the site. If the allergen is still present, a more prolonged form of the same reaction—the so-called late-phase reaction, which lasts a day or two rather than minutes—may supervene. This is a feature of asthmatic attacks in some subjects, in whom repeated episodes also lead to increased sensitivity of the air passages to the constrictive action of histamine. If such persons can escape exposure to the allergen for several weeks, subsequent exposure causes much less severe attacks. A prolonged IgE-induced reaction also causes atopic dermatitis, a skin condition characterized by persistent itching and scaly red patches. These often develop at sites where the skin is bent, such as the elbows and knees. The persistence is due to the influx of mast cells stimulated by the continued presence of the allergen, which is often a harmless substance such as animal hair or dander.

Typical type I allergens

Most people are not unduly susceptible to hay fever or asthma. Those who are—about 10 percent of the population—are sometimes described as atopic (from the term atopy, meaning “uncommon”). Atopic individuals have an increased tendency to make IgE antibodies. This tendency runs in families, though there is no single gene responsible as there is in some hereditary diseases such as hemophilia. Although many innocuous antigens can stimulate a small amount of IgE antibody in the atopic individual, some antigens are much more likely to do so than others, especially if they are repeatedly absorbed in very small amounts through mucosal surfaces. Such antigens are often termed allergens. These substances are usually polypeptides that have carbohydrate groups attached to them. They are resistant to drying, but no special characteristic is known that clearly distinguishes allergens from other antigens. Allergens are present in many types of pollen (which accounts for the seasonal incidence of hay fever), in fungal spores, in animal dander and feathers, in plant seeds (especially when finely ground) and berries, and in what is called house dust. The main allergen in house dust has been identified as the excreta of mites that live on skin scales; other mites (those that live in flour, for example) also excrete potent allergens. This list is far from exhaustive. Sensitivities to chocolate, egg whites, oranges, or cow’s milk are not uncommon.

The amount of allergen needed to trigger an acute type I hypersensitivity reaction in a sensitive person is very small: less than one milligram can produce fatal anaphylaxis if it enters the bloodstream. Medical personnel should inquire about any history of hypersensitivity before administering drugs by injection, and if necessary they should inject a test dose into (rather than through) the skin to ensure that hypersensitivity is absent. In any case, a suitable remedy should be at hand.