Immediate Hypersensitivity

Immediate hypersensitivity can produce allergic rhinitis (chronic runny or stuffy nose); conjunctivitis (red eyes); allergic asthma; atopic dermatitis (urticaria, or hives); and other symptoms. These symptoms result from the immune response to the allergen. In people who are not allergic, the allergen stimulates one type of helper T lymphocyte, the TH1 cells, to secrete interferon-y and inter-leukin-2. In people who are allergic, dendritic cells stimulate the other type of helper T lymphocytes, the TH2 cells, to secrete other lymphokines, including interleukin-4 and interleukin-13. These, in

Table 15.11 Allergy: Comparison of Immediate and Delayed Hypersensitivity

Reactions

Characteristic

Immediate Reaction

Delayed Reaction

Time for onset of symptoms Lymphocytes involved Immune effector

Within several minutes B cells

IgE antibodies

Within 1 to 3 days T cells

Cell-mediated immunity

Allergies most commonly produced

Hay fever, asthma, and most other allergic conditions

Contact dermatitis (such as to poison ivy and poison oak)

Therapy

Antihistamines and adrenergic drugs

Corticosteroids (such as cortisone)

472 Chapter Fifteen

472 Chapter Fifteen

Immediate Hypersensitivity

Mast cell Histamine and other ->■ Allergy chemicals

Mast cell Histamine and other ->■ Allergy chemicals

■ Figure 15.26 The mechanism of immediate hypersensitivity. Allergy (immediate hypersensitivity) is produced when antibodies of the IgE subclass attach to tissue mast cells. The combination of these antibodies with allergens (antigens that provoke an allergic reaction) causes the mast cell to secrete histamine and other chemicals that produce the symptoms of allergy.

turn, stimulate B lymphocytes and plasma cells to secrete antibodies of the IgE subclass instead of the normal IgG antibodies.

Unlike IgG antibodies, IgE antibodies do not circulate in the blood. Instead they attach to tissue mast cells and ba-sophils, which have membrane receptors for these antibodies. When the person is again exposed to the same allergen, the allergen binds to the antibodies attached to the mast cells and basophils. This stimulates these cells to secrete various chemicals, including histamine (fig. 15.26). During this process, leukocytes may also secrete prostaglandin D and related molecules called leukotrienes. These chemicals produce the symptoms of the allergic reactions. It should be noted that his-tamine stimulates smooth muscle contraction in the respiratory tract but causes smooth muscle relaxation in the walls of blood vessels. The different effects are due to differences in the his-tamine receptors of these target tissues.

The symptoms of hay fever (itching, sneezing, tearing, runny nose) are produced largely by histamine and can be treated effectively by antihistamine drugs that block the Hj-histamine receptor. In asthma, the difficulty in breathing is caused by inflammation and smooth muscle constriction in the bronchioles as a result of chemicals released by mast cells and eosinophils. In particular, the bronchoconstriction in asthma is produced by leukotrienes, which are mainly secreted by activated eosinophils. Asthma is treated with epinephrine and more specific P-adrenergic stimulating drugs (chapter 9), which cause bronchodilation, and with corticosteroids, which inhibit inflammation and leukotriene synthesis. Asthma and its treatment are discussed more fully in chapter 16. Regarding food allergies (to milk, eggs, peanuts, soy, wheat, and others), no specific therapy is currently available. People with a food allergy must thus be very diligent about avoiding the particular food.

Clinical Investigation Clues

Remember that Gary's response to the bee sting was greater with the second bee sting than with the first, and was treated with antihistamines.

■ Why was his reaction greater to the second bee sting than to the first?

■ Why were antihistamines useful in treating the effects of the bee sting?

Immediate hypersensitivity to a particular antigen is commonly tested for by injecting various antigens under the skin (fig. 15.27). Within a short time a flare-and-wheal reaction is produced if the person is allergic to that antigen. This reaction is due to the release of histamine and other chemical mediators: the flare (spreading flush) is due to vasodilation, and the wheal (elevated area) results from local edema.

Allergens that provoke immediate hypersensitivity include various foods, bee stings, and pollen grains. The most common allergy of this type is seasonal hay fever, which may be provoked by ragweed (Ambrosia) pollen grains (fig. 15.28a). People who have chronic allergic rhinitis and asthma because of an allergy to dust or feathers are usually allergic to a tiny mite (fig. 15.28b) that lives in dust and eats the scales of skin that are constantly shed from the body. Actually, most of the antigens from the dust mite are not in its body but rather in its feces—tiny particles that can enter the nasal mucosa, much like pollen grains. There can be more than 100,000 mite feces per gram of house dust!

The Immune System

The Immune System

Photos Human Body Mites
(b)
Wheel And Flare Allergy Skin Test
Figure 15.27 A skin test for allergy. If an allergen (a) is injected into the skin of a sensitive individual, a typical flare-and-wheal response (b) occurs within several minutes.
Dust Mite Farinae

■ Figure 15.28 Common allergens. (a) A scanning electron micrograph of ragweed (Ambrosia), which is responsible for hay fever. (b) A scanning electron micrograph of house dust mites (Dermatophagoides farinae). Waste-product particles produced by the dust mite are often responsible for chronic allergic rhinitis and asthma.

Part (a): Reproduced by permission from R. G. Kessel and C. Y. Shih, Scanning Electron Microscopy, Springer-Verlag 1976.

■ Figure 15.28 Common allergens. (a) A scanning electron micrograph of ragweed (Ambrosia), which is responsible for hay fever. (b) A scanning electron micrograph of house dust mites (Dermatophagoides farinae). Waste-product particles produced by the dust mite are often responsible for chronic allergic rhinitis and asthma.

Part (a): Reproduced by permission from R. G. Kessel and C. Y. Shih, Scanning Electron Microscopy, Springer-Verlag 1976.

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Responses

  • segan iggi
    Is the most common type of immediate hypersensitivity.?
    6 years ago
  • Mustafa
    Are the symptoms of immediate hypersensitivity caused by secretion of histamine?
    6 years ago
  • mirabella
    How people nonallergic to an antigen respond?
    6 years ago
  • enrico
    Is wheal and flare is indicative of immediate hypersensitivity or delayed hypersensitivity?
    6 years ago

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