- A type I hypersensitivity (allergic)
reaction between an allergenic antigen and immunoglobulin
E (IgE) bound to mast cells, which stimulates the sudden release of immunological mediators
locally or throughout the body.
- The first symptoms occur within minutes,
and a recurrence may follow hours later (late-stage
response). Anaphylaxis can only occur in an individual previously sensitized to an allergen, as
it is the initial exposure that causes immunoglobulin E (IgE) to bind to mast cells. It is
categorized as local or systemic. Local anaphylactic reactions include hay fever, hives, and
allergic gastroenteritis. Systemic anaphylaxis, which produces peripheral vasodilation,
bronchospasm, and laryngeal edema, can be life- threatening.
ETIOLOGY: IgE antibodies bound to mast cells throughout the body as the result of
previous exposure to an allergenic antigen (sensitization) react when the allergen is
introduced a second time. The mast cells release packets containing chemical
mediators (degranulation) that attract neutrophils and eosinophils and also stimulate
urticaria, vasodilation, increased vascular permeability, and smooth muscle spasm,
esp. in the bronchi and gastrointestinal tract. Chemical mediators involved in
anaphylaxis include histamine, proteases, chemotactic factors, leukotrienes,
prostaglandin D, and cytokines (e.g., TNF-aand interleukins 1, 3, 4, 5, and 6). The
most common agents triggering anaphylaxis are drugs, food, and insect stings. Local
anaphylactic reactions are also commonly triggered by pollens (e.g., hay fever, allergic
rhinitis, allergic asthma).- SYMPTOMS: Local anaphylaxis causes
signs to appear at the site of allergen- antibody
interaction including urticaria (hives), edema, warmth, and erythema. In systemic
anaphylaxis the respiratory tract, cardiovascular system, skin, and gastrointestinal
system are involved. The primary signs are urticaria, angioedema, flushing, wheezing,
dyspnea, increased mucous production, nausea and vomiting, and feelings of
generalized anxiety. Systemic anaphylaxis may be mild or severe enough to cause
shock when massive vasodilation is present.
- TREATMENT: Local anaphylaxis is treated
with antihistamines and occasionally
epinephrine, if the reaction is severe. Treatment for systemic anaphylaxis includes
protection of the airway and administration of oxygen; antihistamines (e.g.,
diphenhydramine or cimetidine to block histamine H1 and H2 receptors); IV fluids to
support blood pressure) and vasopressors (e.g., epinephrine or dopamine) to prevent
or treat shock. Epinephrine also is used to treat bronchospasm. Generally, drugs are
given intravenously; drugs may also be given intramuscularly (e.g., diphenhydramine)
or endotracheally (e.g., epinephrine). In mild cases they may be given subcutaneously.
Corticosteroids may be used to prevent recurrence of bronchospasm and increased
vascular permeability.
- PATIENT CARE: Prevention:
A history of allergic reactions, particularly to drugs,
blood, or contrast media, is obtained. The at- risk patient is observed for reaction
during and immediately after administration of any of these agents. The patient is
taught to identify and avoid common allergens and to recognize an allergic reaction.
Patients also should be taught to wear tags identifying allergies to medications, food, or
insect venom at all times to prevent inappropriate treatment during an emergency.
Individuals who have had an anaphylactic reaction and are unable to avoid future
exposure to allergens should carry a kit containing a syringe of epinephrine and be
taught how to administer it. Patients who are allergic to the venom of Hymenoptera
insects (bees, wasps, hornets) can receive desensitization.
- active
anaphylaxisAnaphylaxis resulting from injection of an antigen.
- exercise-induced
anaphylaxisanaphylactoid reaction.
- local
anaphylaxis A reaction between IgE antibodies bound to mast cells and
an allergen that is limited to a small part of the body.Localized edema and
urticaria (hives) result and may vary in intensity.
- passive
anaphylaxis Anaphylaxis induced by injection of serum from a
sensitized animal into a normal one. After a few hours the latter becomes
sensitized.
- passive
cutaneous anaphylaxis ABBR: PCA.. A laboratory test of antibody
levels in which serum from a sensitized individual is injected into the skin.
Intravenous injection of an antigen accompanied by Evans blue dye at a later
time reacts with the antibodies produced in response to the antigen, creating a
wheal and blue spot at the site, indicating local anaphylaxis.
- systemic
anaphylaxis A reaction between IgE antibodies bound to mast cells
and an allergen that causes the sudden release of immunological mediators in
the skin, respiratory, cardiovascular, and gastrointestinal systems. The
consequences may range from mild (e.g., itching, hives) to life- threatening
(airway obstruction and shock).
 Fig. 5-7
Sequence of events leading to type I hypersensitivity. APC, antigen- presenting
cell; GM- CSF, granulocyte- macrophage colony-stimulating factor; TCR, T-cell receptor;
TH2 cell, CD4+ helper T cell. |
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