Chapter 19

Immunology

1 List the four classic types of hypersensitivity reactions.

 Anaphylactic (type I)

 Cytotoxic (type II)

 Immune complex-mediated (type III)

 Cell-mediated/delayed (type IV)

2 What causes type I hypersensitivity? Give the classic clinical examples.

Type I (anaphylactic) hypersensitivity is a result of preformed IgE antibodies that cause release of vasoactive amines (e.g., histamine, leukotrienes) from mast cells and basophils. Examples are anaphylaxis, atopy, hay fever, urticaria, allergic rhinitis, and some forms of asthma. Anaphylaxis may be caused by bee stings, food allergy (especially peanuts and shellfish), medications (especially penicillins and sulfa drugs), or rubber glove allergy.

3 Describe the clinical findings with chronic type I hypersensitivity.

Look for eosinophilia, elevated IgE levels, positive family history, and seasonal exacerbations. Patients also may have allergic “shiners” (bilateral infraorbital edema) and a transverse nasal crease (caused by frequent nose rubbing). Pale, bluish, edematous nasal turbinates with many eosinophils in clear, watery nasal secretions are also classic.

4 What medication should be avoided in patients with nasal polyps?

Do not give aspirin, which may precipitate a severe asthma attack.

5 How do you recognize and treat true anaphylaxis?

Look for the classic triggers mentioned above just before the patient becomes agitated and flushed and develops itching (urticaria), facial swelling (angioedema), and difficulty in breathing. Symptoms tend to develop rapidly and dramatically.

Treat immediately by securing the airway (laryngeal edema may prevent intubation, in which case do a cricothyroidotomy, if needed) and give subcutaneous epinephrine. Antihistamines are only useful for cutaneous reactions and itching, not for more severe reactions. Use corticosteroids only if the initial treatment options are not available (not a first-line agent).

6 What usually causes hereditary angioedema?

A deficiency of C1 esterase inhibitor (complement) is the usual cause of hereditary angioedema. Patients have diffuse swelling of the lips, eyelids, and possibly the airway, unrelated to allergen exposure. The disease is autosomal dominant; look for a positive family history. C4 complement levels are low. Acute treatment is the same as for anaphylaxis. Androgens are used for long-term treatment because they increase liver production of C1 esterase inhibitor.

7 What type of testing can identify an allergen if it is not obvious?

Skin or patch testing (Fig. 19-1; Plate 46).

8 What causes type II hypersensitivity? List some classic clinical examples.

Type II (cytotoxic) hypersensitivity is a result of preformed IgG and IgM antibodies that react with the antigen and cause secondary inflammation. Examples include the following:

 Autoimmune hemolytic anemia (classically caused by methyldopa, penicillins, or sulfa drugs) or other cytopenias caused by antibodies (e.g., idiopathic thrombocytopenic purpura).

 Transfusion reactions.

 Erythroblastosis fetalis (Rh incompatibility).

 Goodpasture syndrome (watch for linear immunofluorescence on kidney biopsy).

 Myasthenia gravis.

 Graves disease.

 Pernicious anemia.

 Pemphigus vulgaris.

 Hyperacute transplant rejection (as soon as the anastomosis is made at transplant surgery, the transplanted organ deteriorates in front of the surgeon’s eyes).

9 What laboratory test is usually positive with a type II hypersensitivity that causes anemia?

Coombs test (usually the direct Coombs test).

10 What causes type III hypersensitivity? List some classic clinical examples.

Type III (immune complex-mediated) hypersensitivity is caused by antigen-antibody complexes that are usually deposited in vessels and cause an inflammatory response. Examples include serum sickness, lupus erythematosus, rheumatoid arthritis, polyarteritis nodosa, cryoglobulinemia, and certain types of glomerulonephritis (e.g., from chronic hepatitis).

11 What causes type IV hypersensitivity? How is it related to tuberculosis testing?

Type IV (cell-mediated/delayed) hypersensitivity is caused by sensitized T lymphocytes that release inflammatory mediators. The tuberculosis skin test (purified protein derivative [PPD]) exploits this immune system reaction. Other examples include contact dermatitis (especially poison ivy, nickel earrings, cosmetics, and medications), chronic transplant rejection, and granulomas (e.g., sarcoidosis).

12 What sexually transmitted infectious disease should be considered when a patient’s presenting symptoms include a sore throat and mononucleosis-like syndrome?

Human immunodeficiency virus (HIV) infection because initial seroconversion may present as a mononucleosis-like syndrome (e.g., fever, malaise, pharyngitis, rash, lymphadenopathy).

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