Type I hypersensitivity

Anaphylaxis: Prior sensitization has resulted in an immune response initially mediated by CD4 lymphocytes (of the Th2 variety) that promote mast cell proliferation and plasma cell production of IgE. The IgE becomes bound to mast cells in places such as respiratory tract mucosa. Encountering the allergen again leads to mast cell degranulation with release of primary mediators (such as histamine, serotonin) which cause vasodilation, bronchoconstriction, etc. and release of secondary mediators (such as leukotrienes, prostaglandin) which lead to inflammatory cell infiltrates. The process of mast cell degranulation is diagrammed below:

There are two forms of anaphylaxis:

• Systemic anaphylaxis: In some individuals, a severe reaction occurs within minutes, leading to symptomatology such as acute asthma, laryngeal edema, diarrhea, urticaria, and shock. Classic examples are penicillin allergy and bee sting allergy.

• Local anaphylaxis (atopy): About 10% of people have "atopy" and are easily sensitized to allergens that cause a localized reaction when inhaled or ingested. This can produce hay fever, hives, asthma, etc. Classic examples are food allergies and hay fever to ragweed pollen.

Laboratory Findings

• Type 1 hypersensitivity reactions may be accompanied by an increase in eosinophils, as noted with differential count of peripheral white blood cells.

• The serum tryptase may be increased in the hour following mast cell activation.

• Measurement of serum total IgE and levels of specific IgE for certain antigens may be undertaken when allergy therapies are planned. Testing for total or specific IgE should be done only when the history is consistent with allergy and specific allergens are suspected as the cause.

Treatment: A standard adult dose of self-injecting epinephrine is 0.3 mg of 1:1000 epinephrine, which raises blood epinephrine from 0.035 ng/mL at rest to about 10 times that amount, the same as for vigorous exercise. In children, the dose is 0.01 mg per kilogram. Injection is subcutaneous or intramuscular. The injection can be repeated 3 times at 10 minute intervals if indicated. The epinephrine can be life-saving for the acute phase of type I hypersensitivity. Additional therapies including albuterol, antihistamines, and corticosteroids may be indicated for the late phase reaction.