Hypersensitivity Reactions
Help Questions
USMLE Step 1 › Hypersensitivity Reactions
A 9-year-old boy develops an intensely pruritic, linear vesicular rash 2 days after hiking and brushing against poison ivy; vitals are normal and CBC is normal. Which mechanism best describes the patient’s symptoms?
Type II: IgG binds to cell-surface antigens causing complement activation and cell destruction with a positive Coombs test
Nonimmune: Irritant dermatitis from acids causing immediate burning pain without prior sensitization or immune memory
Type IV: Sensitized T cells mediate delayed inflammation and epidermal damage 48–72 hours after exposure to urushiol
Type III: Immune complexes deposit in small vessels causing palpable purpura, arthralgia, and low complement
Type I: Allergen cross-links IgE on mast cells causing immediate wheal-and-flare and possible anaphylaxis within minutes
Explanation
This question tests understanding of hypersensitivity reactions in immunology, specifically the mechanisms and clinical presentations of types I-IV. Hypersensitivity reactions are classified into four types based on the immune mechanism involved, ranging from immediate IgE-mediated responses to delayed T-cell mediated reactions. In this clinical vignette, the patient's symptoms and laboratory findings suggest a type IV hypersensitivity reaction, characterized by delayed vesicular rash after poison ivy exposure. Choice C is correct because it accurately describes the mechanism of a type IV hypersensitivity reaction, as supported by the vignette details including onset 2 days post-exposure. Choice E is incorrect because it confuses nonimmune irritant effects with T-cell mediated responses, which often occurs when students misinterpret timing of symptom onset. To improve understanding, students should focus on the key characteristics and mechanisms of each hypersensitivity type, practice linking clinical presentations with pathophysiological processes, and review case studies that highlight common errors and misconceptions.
A 30-year-old man receives a transfusion and develops fever, flank pain, and hemoglobinuria within 1 hour; labs show low haptoglobin and rising bilirubin. Which mechanism best describes the patient’s symptoms?
Type II: Preformed IgM/IgG against donor RBC antigens activates complement, causing intravascular hemolysis and hemoglobinuria
Type I: IgE-mediated mast-cell activation causing bronchospasm and urticaria without hemoglobinuria or anemia
Nonimmune: Mechanical hemolysis from small-gauge needles causing mild anemia without complement activation
Type III: Immune complexes deposit in joints and kidneys causing low complement and delayed symptoms over days
Type IV: Cytotoxic T cells attack transfused leukocytes causing delayed rash and mucosal lesions
Explanation
This question tests understanding of hypersensitivity reactions in immunology, specifically the mechanisms and clinical presentations of types I-IV. Hypersensitivity reactions are classified into four types based on the immune mechanism involved, ranging from immediate IgE-mediated responses to delayed T-cell mediated reactions. In this clinical vignette, the patient's symptoms and laboratory findings suggest a type II hypersensitivity reaction, characterized by acute hemolytic transfusion reaction with hemoglobinuria. Choice B is correct because it accurately describes the mechanism of a type II hypersensitivity reaction, as supported by the vignette details including low haptoglobin within 1 hour. Choice A is incorrect because it confuses type I allergic responses with transfusion-related hemolysis, which often occurs when students misinterpret intravascular symptoms. To improve understanding, students should focus on the key characteristics and mechanisms of each hypersensitivity type, practice linking clinical presentations with pathophysiological processes, and review case studies that highlight common errors and misconceptions.
A 33-year-old man develops painful oral ulcers and targetoid skin lesions 2 weeks after starting lamotrigine; biopsy shows epidermal necrosis with lymphocytic infiltration. Which mechanism best describes the patient’s symptoms?
Type I: IgE cross-linking triggers mast-cell degranulation causing immediate urticaria and bronchospasm after drug exposure
Type IV: Cytotoxic T cells induce keratinocyte apoptosis, producing delayed severe mucocutaneous reactions after sensitization
Type III: Immune complexes deposit in vessels causing palpable purpura, arthralgia, and low complement levels
Nonimmune: Histamine release from opioids causing flushing and pruritus without epidermal necrosis or mucosal involvement
Type II: IgG binds to cell-surface antigens causing complement-mediated cytotoxicity and a positive direct Coombs test
Explanation
This question tests understanding of hypersensitivity reactions in immunology, specifically the mechanisms and clinical presentations of types I-IV. Hypersensitivity reactions are classified into four types based on the immune mechanism involved, ranging from immediate IgE-mediated responses to delayed T-cell mediated reactions. In this clinical vignette, the patient's symptoms and laboratory findings suggest a type IV hypersensitivity reaction, characterized by delayed mucocutaneous reaction like SJS from lamotrigine. Choice D is correct because it accurately describes the mechanism of a type IV hypersensitivity reaction, as supported by the vignette details including onset 2 weeks later and biopsy findings. Choice A is incorrect because it confuses type I immediate responses with delayed T-cell mediated apoptosis, which often occurs when students misinterpret reaction timing. To improve understanding, students should focus on the key characteristics and mechanisms of each hypersensitivity type, practice linking clinical presentations with pathophysiological processes, and review case studies that highlight common errors and misconceptions.
A 47-year-old woman develops fatigue and pallor after starting a new medication; labs show hemoglobin 7.8 g/dL, reticulocytes 9%, and a positive direct Coombs test. Which of the following best describes the mechanism of the patient’s symptoms?
Immune complexes deposit in glomeruli, causing low complement, proteinuria, and granular immunofluorescence patterns
Antibodies bind RBC surface antigens, leading to complement activation or splenic clearance and hemolytic anemia with Coombs positivity
Th1-mediated macrophage activation causes delayed induration and granuloma formation after antigen exposure over 48–72 hours
Direct marrow suppression reduces erythropoiesis, causing low reticulocytes and pancytopenia rather than immune hemolysis
Allergen-specific IgE triggers mast-cell degranulation, causing immediate bronchospasm and urticaria with elevated serum tryptase
Explanation
This question tests understanding of hypersensitivity reactions in immunology, specifically the mechanisms and clinical presentations of types I-IV. Hypersensitivity reactions are classified into four types based on the immune mechanism involved, ranging from immediate IgE-mediated responses to delayed T-cell mediated reactions. In this clinical vignette, the patient's symptoms and laboratory findings suggest a type II hypersensitivity reaction, characterized by drug-induced hemolytic anemia with positive Coombs and high reticulocytes. Choice A is correct because it accurately describes the mechanism of a type II hypersensitivity reaction, as supported by the vignette details including fatigue and low hemoglobin. Choice E is incorrect because it confuses nonimmune suppression with immune hemolysis, which often occurs when students misinterpret reticulocyte response. To improve understanding, students should focus on the key characteristics and mechanisms of each hypersensitivity type, practice linking clinical presentations with pathophysiological processes, and review case studies that highlight common errors and misconceptions.
A 58-year-old man develops jaundice and anemia after starting methyldopa; labs show elevated LDH, low haptoglobin, and a positive direct Coombs test. Which laboratory finding supports the diagnosis of a type II hypersensitivity reaction?
Positive direct antiglobulin test demonstrating antibodies bound to RBCs, consistent with immune-mediated hemolysis
Low C3 and C4 with granular immune deposits in glomeruli, consistent with immune complex deposition disease
Elevated serum tryptase within 2 hours of symptoms, reflecting mast-cell degranulation after IgE cross-linking
Increased urine eosinophils with acute interstitial nephritis, consistent with drug hypersensitivity affecting tubules
Delayed induration at 72 hours after skin testing, consistent with T-cell mediated delayed hypersensitivity
Explanation
This question tests understanding of hypersensitivity reactions in immunology, specifically the mechanisms and clinical presentations of types I-IV. Hypersensitivity reactions are classified into four types based on the immune mechanism involved, ranging from immediate IgE-mediated responses to delayed T-cell mediated reactions. In this clinical vignette, the patient's symptoms and laboratory findings suggest a type II hypersensitivity reaction, characterized by drug-induced hemolytic anemia with positive Coombs. Choice B is correct because it accurately describes the laboratory finding supporting a type II hypersensitivity reaction, as supported by the vignette details including jaundice and low haptoglobin. Choice C is incorrect because it confuses type III immune complexes with type II direct antibody binding, which often occurs when students misinterpret Coombs test significance. To improve understanding, students should focus on the key characteristics and mechanisms of each hypersensitivity type, practice linking clinical presentations with pathophysiological processes, and review case studies that highlight common errors and misconceptions.
A 25-year-old man develops anaphylaxis minutes after IV contrast; he had a similar reaction previously and now has elevated tryptase. Which of the following best describes the mechanism of the patient’s symptoms?
Direct endothelial toxicity causes hypotension without mast-cell mediators, immune sensitization, or elevated tryptase levels
Immune complexes deposit in tissues causing fever and arthralgias days later with low complement and vasculitis
T-cell mediated keratinocyte apoptosis causes delayed mucosal erosions and epidermal necrosis after 1–3 weeks
IgG binds to RBC antigens causing complement-mediated hemolysis with hemoglobinuria and a positive direct Coombs test
Allergen cross-linking of IgE on mast cells triggers degranulation, causing hypotension and bronchospasm with elevated tryptase
Explanation
This question tests understanding of hypersensitivity reactions in immunology, specifically the mechanisms and clinical presentations of types I-IV. Hypersensitivity reactions are classified into four types based on the immune mechanism involved, ranging from immediate IgE-mediated responses to delayed T-cell mediated reactions. In this clinical vignette, the patient's symptoms and laboratory findings suggest a type I hypersensitivity reaction, characterized by anaphylaxis to IV contrast with elevated tryptase and prior history. Choice A is correct because it accurately describes the mechanism of a type I hypersensitivity reaction, as supported by the vignette details including rapid onset. Choice D is incorrect because it confuses type IV delayed reactions with immediate IgE responses, which often occurs when students misinterpret tryptase elevation. To improve understanding, students should focus on the key characteristics and mechanisms of each hypersensitivity type, practice linking clinical presentations with pathophysiological processes, and review case studies that highlight common errors and misconceptions.
A 60-year-old woman develops petechiae and gingival bleeding 5 days after starting trimethoprim-sulfamethoxazole; platelets are 8,000/µL and hemoglobin is normal. What type of hypersensitivity reaction is most likely responsible?
Nonimmune: Bone marrow suppression causing pancytopenia with low platelets, anemia, and neutropenia simultaneously
Type III: Immune complex deposition causing fever, arthralgia, and low complement several days after exposure
Type IV: T-cell mediated dermatitis causing localized vesicles at contact sites 48–72 hours after exposure
Type II: Drug-dependent antibodies target platelets, causing immune thrombocytopenia and bleeding with severe thrombocytopenia
Type I: IgE-mediated mast-cell activation causing urticaria and bronchospasm within minutes of drug exposure
Explanation
This question tests understanding of hypersensitivity reactions in immunology, specifically the mechanisms and clinical presentations of types I-IV. Hypersensitivity reactions are classified into four types based on the immune mechanism involved, ranging from immediate IgE-mediated responses to delayed T-cell mediated reactions. In this clinical vignette, the patient's symptoms and laboratory findings suggest a type II hypersensitivity reaction, characterized by drug-induced thrombocytopenia with bleeding. Choice B is correct because it accurately describes the mechanism of a type II hypersensitivity reaction, as supported by the vignette details including isolated low platelets. Choice E is incorrect because it confuses nonimmune marrow suppression with antibody-mediated destruction, which often occurs when students misinterpret selective cytopenia. To improve understanding, students should focus on the key characteristics and mechanisms of each hypersensitivity type, practice linking clinical presentations with pathophysiological processes, and review case studies that highlight common errors and misconceptions.
A 44-year-old woman develops fever, urticaria, arthralgias, and lymphadenopathy 8 days after starting cefaclor; C3 is low and urinalysis shows mild proteinuria. What type of hypersensitivity reaction is most likely responsible?
Type I: IgE-mediated mast-cell activation causing immediate bronchospasm and hypotension within minutes of exposure
Type II: Antibody-mediated cytotoxicity against RBCs causing hemolysis and a positive direct Coombs test
Nonimmune: Viral exanthem causing rash and fever without complement consumption or immune complex formation
Type IV: T-cell mediated contact dermatitis causing localized vesicles and pruritus 48–72 hours after exposure
Type III: Immune complex deposition causing serum sickness features with hypocomplementemia and systemic symptoms
Explanation
This question tests understanding of hypersensitivity reactions in immunology, specifically the mechanisms and clinical presentations of types I-IV. Hypersensitivity reactions are classified into four types based on the immune mechanism involved, ranging from immediate IgE-mediated responses to delayed T-cell mediated reactions. In this clinical vignette, the patient's symptoms and laboratory findings suggest a type III hypersensitivity reaction, characterized by serum sickness from cefaclor with low C3. Choice C is correct because it accurately describes the mechanism of a type III hypersensitivity reaction, as supported by the vignette details including fever and arthralgias 8 days later. Choice D is incorrect because it confuses type IV contact dermatitis with systemic immune complex disease, which often occurs when students misinterpret complement levels. To improve understanding, students should focus on the key characteristics and mechanisms of each hypersensitivity type, practice linking clinical presentations with pathophysiological processes, and review case studies that highlight common errors and misconceptions.
A 38-year-old woman with SLE has worsening proteinuria; kidney biopsy shows granular deposits of IgG and C3 along the glomerular basement membrane. Which laboratory finding supports the diagnosis of a type III hypersensitivity reaction?
Elevated serum tryptase shortly after symptoms, consistent with mast-cell degranulation in anaphylaxis
Positive direct Coombs test demonstrating antibodies bound to RBCs, consistent with autoimmune hemolytic anemia
Delayed induration 72 hours after PPD placement, consistent with T-cell mediated delayed hypersensitivity
Low serum complement levels due to consumption during immune complex activation, consistent with type III hypersensitivity
Elevated creatine kinase with muscle weakness, consistent with inflammatory myopathy rather than immune complex disease
Explanation
This question tests understanding of hypersensitivity reactions in immunology, specifically the mechanisms and clinical presentations of types I-IV. Hypersensitivity reactions are classified into four types based on the immune mechanism involved, ranging from immediate IgE-mediated responses to delayed T-cell mediated reactions. In this clinical vignette, the patient's symptoms and laboratory findings suggest a type III hypersensitivity reaction, characterized by immune complex deposition in SLE nephritis with granular deposits. Choice A is correct because it accurately describes the laboratory finding supporting a type III hypersensitivity reaction, as supported by the vignette details including worsening proteinuria. Choice B is incorrect because it confuses type II hemolysis with type III complexes, which often occurs when students misinterpret biopsy patterns. To improve understanding, students should focus on the key characteristics and mechanisms of each hypersensitivity type, practice linking clinical presentations with pathophysiological processes, and review case studies that highlight common errors and misconceptions.
A 62-year-old man develops anemia and mild jaundice after 2 weeks of ceftriaxone; labs show elevated indirect bilirubin and a positive direct Coombs test. What type of hypersensitivity reaction is most likely responsible?
Type IV: T-cell mediated dermatitis causing localized rash 48–72 hours after contact with an allergen
Type I: IgE-mediated mast-cell degranulation causing immediate urticaria and airway edema minutes after exposure
Type II: Antibody-mediated destruction of RBCs triggered by drug exposure, producing Coombs-positive hemolytic anemia
Nonimmune: Microangiopathic hemolysis causing schistocytes and thrombocytopenia without a positive Coombs test
Type III: Immune complex deposition causing fever, arthralgia, and low complement with delayed systemic symptoms
Explanation
This question tests understanding of hypersensitivity reactions in immunology, specifically the mechanisms and clinical presentations of types I-IV. Hypersensitivity reactions are classified into four types based on the immune mechanism involved, ranging from immediate IgE-mediated responses to delayed T-cell mediated reactions. In this clinical vignette, the patient's symptoms and laboratory findings suggest a type II hypersensitivity reaction, characterized by drug-induced hemolytic anemia from ceftriaxone with positive Coombs. Choice B is correct because it accurately describes the mechanism of a type II hypersensitivity reaction, as supported by the vignette details including anemia after 2 weeks. Choice E is incorrect because it confuses nonimmune hemolysis with antibody-mediated destruction, which often occurs when students misinterpret Coombs positivity. To improve understanding, students should focus on the key characteristics and mechanisms of each hypersensitivity type, practice linking clinical presentations with pathophysiological processes, and review case studies that highlight common errors and misconceptions.