Review Question - QID 109089

This patient with cough and shortness of breath who presents with wheezing and eczema most likely has asthma, which is a type I hypersensitivity reaction.

Type I hypersensitivity is caused by antigen cross-linking of IgE on sensitized mast cells. This causes the release of vasoactive mediators (e.g., histamine, leukotrienes, and prostaglandin D2) and inflammatory mediators (e.g., TNF-alpha and leukotriene B4). This mechanism is responsible for asthma, allergic rhinitis (hay fever), anaphylaxis, angioedema, urticaria, and allergies to medications and food. Asthma, in particular, is characterized by cough and shortness of breath that is worse at night and associated with end-expiratory wheezing.

Figure A demonstrates the characteristic appearance of an atopic dermatitis rash. These rashes frequently present as erythematous plaques and papules around the flexor surfaces of the elbows and knees. Excoriations, scaling, and vesicles can sometimes be seen. Of note, infants often have lesions on the cheeks, scalp, and extensor surfaces while older children and adults have lesions on the flexor surfaces.

Incorrect Answers:
Answers 2-3: Type II hypersensitivity reactions are antibody-mediated. In these reactions, IgM and IgG bind to antigen to activate complement. These can either be cytotoxic or non-cytotoxic depending on the target of the antibodies. Examples of cytotoxic conditions caused by type II hypersensitivity reactions are autoimmune hemolytic anemia (anti-RBC antibodies), idiopathic thrombocytopenic purpura (anti-platelet antibodies), and hemolytic disease of the newborn (anti-RhD+ antibodies). Examples of non-cytotoxic conditions caused by type II hypersensitivity reactions are Graves’ disease (anti-TSH receptor antibodies) and myasthenia gravis (anti-acetylcholine receptor antibodies).

Answer 4: Type III hypersensitivity reactions are immune complex-mediated. In these reactions, antigen-antibody complexes deposit in healthy tissues, initiating localized inflammatory responses in these tissues. Examples of conditions caused by type III hypersensitivity reactions are systemic lupus erythematosus, serum sickness, and post-streptococcal glomerulonephritis.

Answer 5: Type IV hypersensitivity reactions are cell-mediated. In these reactions, T-cells are sensitized to an antigen. On subsequent exposure to that antigen, effector T-cells and macrophages will be activated. Examples of type IV hypersensitivity reactions include type I diabetes mellitus, multiple sclerosis, and Hashimoto’s thyroiditis.

Bullet Summary:
Type I hypersensitivity reactions are rapid IgE-mediated reactions that lead to mast cell degranulation and examples include allergic rhinitis, asthma, and anaphylaxis.