A 40-year-old woman with myasthenia gravis on pyridostigmine develops worsening weakness, diplopia, and dysphagia. She recently had URI and received azithromycin. Exam shows bilateral ptosis, ophthalmoplegia, and proximal muscle weakness with preserved reflexes. Her acetylcholinesterase inhibitor dose was increased 3 days ago. Edrophonium test shows no improvement. ABG shows hypercapnia. Evaluate the synthesis of clinical findings and determine the life-threatening complication requiring immediate intervention.
Q2
A 32-year-old pregnant woman at 20 weeks gestation with known anti-Rh(D) antibodies from previous pregnancy presents for routine prenatal care. Her current fetus is Rh(D)-positive by cell-free DNA testing. Middle cerebral artery Doppler shows increased peak systolic velocity. Fetal ultrasound reveals polyhydramnios and ascites. Amniocentesis shows elevated bilirubin. Synthesize the pathophysiology and evaluate the therapeutic intervention that addresses the underlying immune mechanism.
Q3
A 25-year-old man with HIV (CD4 count 450 cells/μL) on antiretroviral therapy for 6 months develops fever, lymphadenopathy, and worsening respiratory symptoms. Chest CT shows new mediastinal lymphadenopathy and pulmonary infiltrates. Sputum is positive for Mycobacterium tuberculosis. His viral load is undetectable. Evaluate the immunologic phenomenon responsible for his clinical deterioration despite virologic control.
Q4
A 55-year-old man with chronic hepatitis C develops palpable purpura on lower extremities, arthralgias, and weakness. Labs show elevated creatinine (2.5 mg/dL), low C4 (8 mg/dL), normal C3, positive rheumatoid factor, and cryoglobulins. Skin biopsy shows leukocytoclastic vasculitis. Renal biopsy reveals membranoproliferative glomerulonephritis with subendothelial deposits. Analyze the pathophysiologic link between the viral infection and systemic manifestations.
Q5
A 3-month-old infant presents with failure to thrive, chronic diarrhea, and oral candidiasis. Physical exam reveals absent tonsils and lymph nodes. Chest X-ray shows no thymic shadow. Flow cytometry reveals CD3+ T cells <200/μL, normal CD19+ B cells, and normal NK cells. Serum immunoglobulins are low despite normal B cell counts. Analyze the immunologic defect and predict the long-term complications.
Q6
A 42-year-old woman with Graves' disease treated with radioactive iodine develops periorbital edema, proptosis, and diplopia. TSH is suppressed, but free T4 is normal. Anti-TSH receptor antibodies are elevated. MRI shows enlarged extraocular muscles with increased retro-orbital fat. Analyze the immunopathologic mechanism causing her ophthalmopathy despite thyroid function normalization.
Q7
A 35-year-old woman receives a kidney transplant from her HLA-matched sibling. She is maintained on tacrolimus and mycophenolate. Three months post-transplant, her creatinine rises from 1.0 to 2.8 mg/dL over 2 weeks. Biopsy shows tubulitis with mononuclear cell infiltration and tubular epithelial damage. C4d staining is negative. Apply transplant immunology to determine the rejection mechanism.
Q8
A 6-year-old boy presents with recurrent sinopulmonary infections since age 2. He has had three episodes of pneumococcal pneumonia and two episodes of H. influenzae meningitis despite appropriate vaccinations. Serum studies show low IgG (200 mg/dL), low IgA (10 mg/dL), and low IgM (15 mg/dL). Flow cytometry shows absent CD19+ B cells but normal CD3+ T cells. Apply this immunologic profile to determine the underlying defect.
Q9
A 45-year-old man with rheumatoid arthritis on methotrexate develops progressive dyspnea and dry cough. Chest CT shows bilateral ground-glass opacities. Bronchoalveolar lavage reveals CD8+ T lymphocytes. Transbronchial biopsy shows non-caseating granulomas with multinucleated giant cells. Apply immunologic principles to identify the hypersensitivity reaction causing his pulmonary symptoms.
Q10
A 28-year-old woman presents with a butterfly rash, photosensitivity, and joint pain for 3 months. Laboratory studies show positive ANA (1:640), anti-dsDNA antibodies, and decreased C3 and C4 complement levels. Urinalysis reveals proteinuria (3+) and RBC casts. Apply your understanding of immune complex-mediated disease to determine the primary pathologic mechanism.
Immunopathology US Medical PG Practice Questions and MCQs
Question 1: A 40-year-old woman with myasthenia gravis on pyridostigmine develops worsening weakness, diplopia, and dysphagia. She recently had URI and received azithromycin. Exam shows bilateral ptosis, ophthalmoplegia, and proximal muscle weakness with preserved reflexes. Her acetylcholinesterase inhibitor dose was increased 3 days ago. Edrophonium test shows no improvement. ABG shows hypercapnia. Evaluate the synthesis of clinical findings and determine the life-threatening complication requiring immediate intervention.
A. Botulism from contaminated food requiring antitoxin administration
B. Lambert-Eaton syndrome from paraneoplastic antibodies requiring immunosuppression
C. Myasthenic crisis from insufficient acetylcholinesterase inhibition requiring increased pyridostigmine
D. Cholinergic crisis from excessive acetylcholinesterase inhibition requiring drug cessation and atropine (Correct Answer)
E. Guillain-Barré syndrome from post-infectious autoimmunity requiring plasmapheresis
Explanation: ***Cholinergic crisis from excessive acetylcholinesterase inhibition requiring drug cessation and atropine***
- A **negative edrophonium test** and recent increase in **pyridostigmine** dose indicates that nicotinic receptors are overstimulated and desensitized, preventing clinical improvement with further drug administration.
- The presence of life-threatening **respiratory failure (hypercapnia)** due to excessive **cholinergic stimulation** necessitates the cessation of the offending drug and the use of **atropine** to manage muscarinic side effects.
*Botulism from contaminated food requiring antitoxin administration*
- While it causes **descending paralysis**, its onset is typically associated with ingestion of C. botulinum toxin and involves **fixed dilated pupils**, unlike the pinpoint pupils often seen in cholinergic excess.
- It would not explain why a patient already on therapy for **myasthenia gravis** is experiencing a crisis immediately following a medication dose increase.
*Lambert-Eaton syndrome from paraneoplastic antibodies requiring immunosuppression*
- This condition involves **presynaptic** calcium channel antibodies and typically shows **improvement with muscle use**, the opposite of this patient's presentation.
- It is generally associated with **small cell lung cancer** and would not cause an acute respiratory failure triggered by a recent change in pyridostigmine dosing.
*Myasthenic crisis from insufficient acetylcholinesterase inhibition requiring increased pyridostigmine*
- A **myasthenic crisis** would typically show clinical improvement during an **edrophonium (Tensilon) test** as it addresses the lack of acetylcholine at the junction.
- Although triggered by infections or drugs like **azithromycin**, the lack of response to edrophonium after a recent dose increase strongly points toward over-medication rather than under-medication.
*Guillain-Barré syndrome from post-infectious autoimmunity requiring plasmapheresis*
- **Guillain-Barré syndrome** typically presents with **ascending paralysis** and a hallmark feature of **absent deep tendon reflexes**, whereas this patient has preserved reflexes.
- It does not present with **ophthalmoplegia** or ptosis as early or prominent signs compared to the sudden clinical worsening seen in established myasthenia cases.
Question 2: A 32-year-old pregnant woman at 20 weeks gestation with known anti-Rh(D) antibodies from previous pregnancy presents for routine prenatal care. Her current fetus is Rh(D)-positive by cell-free DNA testing. Middle cerebral artery Doppler shows increased peak systolic velocity. Fetal ultrasound reveals polyhydramnios and ascites. Amniocentesis shows elevated bilirubin. Synthesize the pathophysiology and evaluate the therapeutic intervention that addresses the underlying immune mechanism.
A. Immediate delivery and phototherapy to prevent kernicterus
B. Administration of anti-Rh(D) immunoglobulin to mother to neutralize antibodies
C. Intrauterine transfusion to replace hemolyzed fetal erythrocytes
D. Maternal high-dose IVIG to competitively inhibit FcRn-mediated antibody transport (Correct Answer)
E. Maternal plasmapheresis to remove anti-Rh(D) antibodies
Explanation: ***Maternal high-dose IVIG to competitively inhibit FcRn-mediated antibody transport***
- This therapy directly addresses the underlying **immune mechanism** by blocking the **neonatal Fc receptor (FcRn)** in the placenta, which is responsible for transporting maternal IgG to the fetus.
- By inhibiting this transport, it reduces the quantity of **anti-Rh(D) antibodies** reaching the fetal circulation, thereby mitigating **Type II hypersensitivity** destruction of fetal RBCs.
*Immediate delivery and phototherapy to prevent kernicterus*
- Immediate delivery at **20 weeks gestation** is inappropriate as the fetus is pre-viable and suffers from extreme **prematurity**.
- Phototherapy is a neonatal treatment for **hyperbilirubinemia** and does not address the active **in utero hemolysis** or immune transport mechanism.
*Administration of anti-Rh(D) immunoglobulin to mother to neutralize antibodies*
- **Rho(D) immune globulin** is used for **prophylaxis** in unsensitized Rh-negative women to prevent the initial immune response.
- It is ineffective once a mother is already **alloimmunized** (sensitized), as it cannot neutralize a high titer of pre-existing memory B-cell mediated antibody production.
*Intrauterine transfusion to replace hemolyzed fetal erythrocytes*
- While **intrauterine transfusion (IUT)** is the standard treatment for fetal anemia, it addresses the **consequence** (anemia) rather than the underlying immune transport mechanism.
- It is often performed when **Middle Cerebral Artery (MCA) Doppler** shows high peak systolic velocity, signaling severe anemia, but does not stop the maternal antibodies from entering fetal blood.
*Maternal plasmapheresis to remove anti-Rh(D) antibodies*
- **Plasmapheresis** can acutely lower maternal antibody titers but its effects are temporary due to rapid **rebound antibody production**.
- It is generally reserved for very early, severe cases and is less effective at targeting the specific **placental transport** mechanism than IVIG.
Question 3: A 25-year-old man with HIV (CD4 count 450 cells/μL) on antiretroviral therapy for 6 months develops fever, lymphadenopathy, and worsening respiratory symptoms. Chest CT shows new mediastinal lymphadenopathy and pulmonary infiltrates. Sputum is positive for Mycobacterium tuberculosis. His viral load is undetectable. Evaluate the immunologic phenomenon responsible for his clinical deterioration despite virologic control.
A. Opportunistic infection from progressive immunosuppression
B. Development of antiretroviral drug resistance causing treatment failure
C. Paradoxical reaction from inadequate tuberculosis treatment
D. Immune reconstitution inflammatory syndrome from restored pathogen-specific responses (Correct Answer)
E. Drug-induced hypersensitivity pneumonitis from antiretroviral therapy
Explanation: ***Immune reconstitution inflammatory syndrome from restored pathogen-specific responses***
- **Immune reconstitution inflammatory syndrome (IRIS)** occurs when antiretroviral therapy (ART) leads to a rapid recovery of **CD4+ T cells**, which then mount an exaggerated inflammatory response against a pre-existing pathogen.
- This patient demonstrates **unmasking IRIS**, where an underlying subclinical **Mycobacterium tuberculosis** infection becomes symptomatic due to the restoration of **pathogen-specific immune responses** despite a low viral load.
*Opportunistic infection from progressive immunosuppression*
- This is unlikely because the patient's **viral load is undetectable** and the **CD4 count is 450**, indicating successful immune recovery rather than failure.
- True **opportunistic infections** typically occur when the immune system is severely depleted, not when it is reconstituting under ART.
*Development of antiretroviral drug resistance causing treatment failure*
- **Resistance** would typically present with a **rising viral load** and a dropping CD4 count, which is the opposite of this patient's laboratory findings.
- Clinical deterioration due to resistance is a slow process of **viral escape**, not an acute inflammatory presentation with new lymphadenopathy.
*Paradoxical reaction from inadequate tuberculosis treatment*
- While **paradoxical reactions** occur during TB therapy, this patient was not previously treated for TB; the symptoms developed after starting **ART** for HIV.
- This scenario specifically describes an **unmasking IRIS** event rather than a reaction to existing anti-tubercular chemotherapy.
*Drug-induced hypersensitivity pneumonitis from antiretroviral therapy*
- While ART drugs can have side effects, **hypersensitivity pneumonitis** would not explain the presence of **Mycobacterium tuberculosis** in the sputum.
- The focal **mediastinal lymphadenopathy** and positive sputum cultures point towards an infection-driven inflammatory process rather than a drug allergy.
Question 4: A 55-year-old man with chronic hepatitis C develops palpable purpura on lower extremities, arthralgias, and weakness. Labs show elevated creatinine (2.5 mg/dL), low C4 (8 mg/dL), normal C3, positive rheumatoid factor, and cryoglobulins. Skin biopsy shows leukocytoclastic vasculitis. Renal biopsy reveals membranoproliferative glomerulonephritis with subendothelial deposits. Analyze the pathophysiologic link between the viral infection and systemic manifestations.
A. Molecular mimicry between viral proteins and vascular antigens triggering autoimmunity
B. Direct viral cytopathic effect on endothelial cells causing vasculitis
C. HCV suppression of regulatory T cells causing loss of self-tolerance
D. HCV-induced production of cryoglobulins forming immune complexes that deposit in vessels (Correct Answer)
E. Viral integration into host genome causing malignant transformation of B cells
Explanation: ***HCV-induced production of cryoglobulins forming immune complexes that deposit in vessels***
- In **Type II Mixed Cryoglobulinemia**, chronic **HCV infection** provides a continuous antigenic stimulus that drives monoclonal **B-cell expansion**, producing **IgM rheumatoid factor**.
- These IgM-IgG **immune complexes** deposit in small-to-medium vessels, causing a **Type III hypersensitivity** reaction that results in **leukocytoclastic vasculitis**, low **C4 levels**, and **MPGN**.
*Molecular mimicry between viral proteins and vascular antigens triggering autoimmunity*
- **Molecular mimicry** involves cross-reactivity between foreign and self-antigens, typically seen in conditions like **Rheumatic Heart Disease**.
- This mechanism does not explain the presence of **cryoglobulins** or the selective **complement C4 consumption** seen in this patient.
*Direct viral cytopathic effect on endothelial cells causing vasculitis*
- A **direct cytopathic effect** implies the virus itself replicates within and destroys host cells, which is not the primary driver of HCV-related systemic vasculitis.
- The systemic damage in HCV cryoglobulinemia is mediated by the **host’s immune response** (immune complexes) rather than direct viral invasion of the vasculature.
*HCV suppression of regulatory T cells causing loss of self-tolerance*
- While chronic infections can modulate **T-cell function**, the primary pathophysiology of cryoglobulinemia is a **B-cell proliferative** disorder and complex deposition.
- Generalized **loss of self-tolerance** usually presents with broad-spectrum autoimmune diseases rather than specific **cold-precipitating immune complexes**.
*Viral integration into host genome causing malignant transformation of B cells*
- **HCV** is an RNA virus that stays in the cytoplasm and does not **integrate into the host genome**, unlike DNA viruses or Retroviruses.
- Most cases of cryoglobulinemia are **benign B-cell expansions**, though they can occasionally progress to **B-cell non-Hodgkin lymphoma** over many years.
Question 5: A 3-month-old infant presents with failure to thrive, chronic diarrhea, and oral candidiasis. Physical exam reveals absent tonsils and lymph nodes. Chest X-ray shows no thymic shadow. Flow cytometry reveals CD3+ T cells <200/μL, normal CD19+ B cells, and normal NK cells. Serum immunoglobulins are low despite normal B cell counts. Analyze the immunologic defect and predict the long-term complications.
A. Phagocyte deficiency requiring prophylactic antibiotics with normal longevity
B. Pure B cell deficiency requiring lifelong IVIG with normal life expectancy
C. Isolated T cell deficiency with preserved humoral immunity requiring antivirals only
D. Combined T and B cell dysfunction requiring bone marrow transplant to prevent death from opportunistic infections (Correct Answer)
E. Transient immunodeficiency that resolves spontaneously by age 2 years
Explanation: ***Combined T and B cell dysfunction requiring bone marrow transplant to prevent death from opportunistic infections***
- The patient exhibits features of **Severe Combined Immunodeficiency (SCID)**, specifically a **T-B+NK+ phenotype**, where profound T-cell lymphopenia leads to functional B-cell failure and low **serum immunoglobulins**.
- Without a **hematopoietic stem cell transplant**, this condition is fatal within the first year of life due to severe **opportunistic infections** like **Pneumocystis jirovecii** and **Candida**.
*Phagocyte deficiency requiring prophylactic antibiotics with normal longevity*
- Phagocyte disorders like **Chronic Granulomatous Disease** usually present with **catalase-positive** bacterial and fungal abscesses, not a complete absence of the **thymic shadow**.
- Flow cytometry in phagocytic defects would typically show normal **CD3+ T cell** and **CD19+ B cell** counts.
*Pure B cell deficiency requiring lifelong IVIG with normal life expectancy*
- **X-linked agammaglobulinemia (XLA)** presents with absent or low counts of **CD19+ B cells**, whereas this patient has a normal B-cell count.
- XLA does not explain the **absent thymic shadow** or the severe **T-cell lymphopenia** (<200/μL) seen in this infant.
*Isolated T cell deficiency with preserved humoral immunity requiring antivirals only*
- While **T-cell deficiency** is primary, humoral immunity is never preserved because **T-cell help** (via CD40L and cytokines) is essential for **B-cell class switching**.
- Antivirals alone are insufficient; patients require **immune reconstitution** through transplantation to survive beyond infancy.
*Transient immunodeficiency that resolves spontaneously by age 2 years*
- **Transient hypogammaglobulinemia of infancy** involves a delay in antibody production but features normal **T-cell counts** and a present **thymic shadow**.
- The severity of clinical signs, such as **failure to thrive** and **absent lymphoid tissue**, rules out a self-limiting or transient condition.
Question 6: A 42-year-old woman with Graves' disease treated with radioactive iodine develops periorbital edema, proptosis, and diplopia. TSH is suppressed, but free T4 is normal. Anti-TSH receptor antibodies are elevated. MRI shows enlarged extraocular muscles with increased retro-orbital fat. Analyze the immunopathologic mechanism causing her ophthalmopathy despite thyroid function normalization.
A. Immune complex deposition from circulating thyroid hormone-antibody complexes
B. Continued TSH receptor stimulation on orbital fibroblasts by thyroid-stimulating antibodies (Correct Answer)
C. Type IV hypersensitivity from T-cell infiltration of orbital tissue
D. Cross-reactive antibodies targeting thyroglobulin in extraocular muscles
E. Secondary hyperparathyroidism causing calcium deposition in retro-orbital space
Explanation: ***Continued TSH receptor stimulation on orbital fibroblasts by thyroid-stimulating antibodies***
- Graves' ophthalmopathy is driven by **thyroid-stimulating immunoglobulins (TSI)** binding to **TSH receptors** expressed on orbital **fibroblasts** and adipocytes.
- This activation triggers the production of **glycosaminoglycans** (like hyaluronic acid), leading to osmotic edema, muscle swelling, and increased retro-orbital pressure independent of the thyroid's metabolic state.
*Immune complex deposition from circulating thyroid hormone-antibody complexes*
- This describes a **Type III hypersensitivity** mechanism, which is not the underlying cause of Graves' eye disease.
- Thyroid hormones are not typically part of circulating **immune complexes** that deposit in orbital tissues to cause inflammation.
*Type IV hypersensitivity from T-cell infiltration of orbital tissue*
- While **T-cells** are present in the orbital infiltrate and release cytokines, they are secondary to the primary **antibody-mediated (Type II)** activation of receptors.
- Type IV hypersensitivity focuses on cell-mediated destruction, whereas this condition is characterized by **receptor-mediated stimulation** and tissue expansion.
*Cross-reactive antibodies targeting thyroglobulin in extraocular muscles*
- While thyroglobulin can be detected in orbital tissues, it is not the primary antigen driving **proptosis** and muscle enlargement.
- The pathology is specifically linked to the **TSH receptor** shared between the thyroid gland and the orbit, rather than cross-reactivity with **thyroglobulin**.
*Secondary hyperparathyroidism causing calcium deposition in retro-orbital space*
- **Secondary hyperparathyroidism** occurs in response to low vitamin D or chronic kidney disease and does not cause **Graves' ophthalmopathy**.
- The retro-orbital expansion in Graves' disease is due to **soft tissue hypertrophy** and fluid accumulation, not **dystrophic or metastatic calcification**.
Question 7: A 35-year-old woman receives a kidney transplant from her HLA-matched sibling. She is maintained on tacrolimus and mycophenolate. Three months post-transplant, her creatinine rises from 1.0 to 2.8 mg/dL over 2 weeks. Biopsy shows tubulitis with mononuclear cell infiltration and tubular epithelial damage. C4d staining is negative. Apply transplant immunology to determine the rejection mechanism.
A. Acute antibody-mediated rejection with complement activation
B. Hyperacute rejection from preformed antibodies against donor HLA
C. Chronic rejection from progressive arteriopathy
D. Acute T-cell mediated rejection from alloreactive cytotoxic T cells (Correct Answer)
E. Drug-induced interstitial nephritis from tacrolimus toxicity
Explanation: ***Acute T-cell mediated rejection from alloreactive cytotoxic T cells***
- This mechanism is characterized histologically by **tubulitis** and **mononuclear cell infiltration**, typically occurring weeks to months after transplantation.
- It is driven by **host T lymphocytes** (CD8+ and CD4+) that react against donor MHC antigens, leading to direct **parenchymal destruction** regardless of HLA matching.
*Acute antibody-mediated rejection with complement activation*
- This diagnosis requires **C4d deposition** in the peritubular capillaries, which acts as a marker for the classical complement pathway activation by antibodies.
- The biopsy in this case specifically notes that **C4d staining is negative**, suggesting a cellular rather than humoral mechanism.
*Hyperacute rejection from preformed antibodies against donor HLA*
- This occurs within **minutes to hours** of anastomosis due to preformed **anti-donor antibodies**, leading to widespread thrombosis and fibrinoid necrosis.
- The clinical timeline of **three months** post-transplant and the gradual rise in creatinine rule out this immediate type of rejection.
*Chronic rejection from progressive arteriopathy*
- Chronic rejection occurs over **months to years** and is histologically defined by **interstitial fibrosis**, tubular atrophy, and vascular intimal thickening.
- The presence of active **tubulitis** and a sharp increase in creatinine over just two weeks points toward an acute process rather than chronic remodeling.
*Drug-induced interstitial nephritis from tacrolimus toxicity*
- Tacrolimus nephrotoxicity usually presents with **isometric vacuolization** of tubular cells or **arteriolar hyalinosis** rather than active tubulitis.
- While it causes a rise in creatinine, it lacks the dense **mononuclear infiltration** within the tubular epithelium characteristic of T-cell mediated rejection.
Question 8: A 6-year-old boy presents with recurrent sinopulmonary infections since age 2. He has had three episodes of pneumococcal pneumonia and two episodes of H. influenzae meningitis despite appropriate vaccinations. Serum studies show low IgG (200 mg/dL), low IgA (10 mg/dL), and low IgM (15 mg/dL). Flow cytometry shows absent CD19+ B cells but normal CD3+ T cells. Apply this immunologic profile to determine the underlying defect.
A. Defective NADPH oxidase in phagocytes
B. Defective tyrosine kinase preventing B cell maturation (Correct Answer)
C. Absent CD40 ligand on activated T cells
D. Absent thymic shadow with T cell lymphopenia
E. Defective adenosine deaminase causing lymphocyte toxicity
Explanation: ***Defective tyrosine kinase preventing B cell maturation***
- This patient has **X-linked agammaglobulinemia (Bruton's)**, characterized by a mutation in **Bruton Tyrosine Kinase (BTK)**, which prevents **pre-B cells** from maturing into mature **B cells (CD19+)**.
- The clinical presentation of recurrent infections with **encapsulated organisms** (Pneumococcus, H. influenzae) and **pan-hypogammaglobulinemia** typically manifests after **maternal IgG** wanes (around 6 months of age).
*Defective NADPH oxidase in phagocytes*
- This is the underlying defect in **Chronic Granulomatous Disease (CGD)**, which leads to impaired killing of **catalase-positive organisms** (e.g., S. aureus, Aspergillus).
- Patients with CGD have normal levels of B and T cells and do not present with the global **hypogammaglobulinemia** seen in this case.
*Absent CD40 ligand on activated T cells*
- This defect causes **Hyper-IgM Syndrome**, where B cells cannot undergo **class-switching**, leading to high or normal IgM but absent IgG, IgA, and IgE.
- Unlike the patient in the scenario, flow cytometry would show a **normal number of CD19+ B cells**, though they are functionally immature.
*Absent thymic shadow with T cell lymphopenia*
- An **absent thymic shadow** is characteristic of **DiGeorge Syndrome** (22q11 deletion) or **SCID**, both of which involve profound **T-cell (CD3+) deficiency**.
- This patient has a **normal CD3+ T cell count**, which rules out primary thymic or pan-lymphocyte aplasia.
*Defective adenosine deaminase causing lymphocyte toxicity*
- This describes **Adenosine Deaminase (ADA) deficiency**, a common cause of **Autosomal Recessive SCID**.
- This condition leads to the accumulation of **toxic metabolites** that kill both **B cells and T cells**, whereas this patient has preserved T-cell numbers.
Question 9: A 45-year-old man with rheumatoid arthritis on methotrexate develops progressive dyspnea and dry cough. Chest CT shows bilateral ground-glass opacities. Bronchoalveolar lavage reveals CD8+ T lymphocytes. Transbronchial biopsy shows non-caseating granulomas with multinucleated giant cells. Apply immunologic principles to identify the hypersensitivity reaction causing his pulmonary symptoms.
A. Type III hypersensitivity from methotrexate-protein immune complexes
B. Type I hypersensitivity from methotrexate-induced IgE production
C. Type IV hypersensitivity from drug-induced delayed T-cell response (Correct Answer)
D. Type II hypersensitivity from anti-basement membrane antibodies
E. Mixed Type II and III hypersensitivity with complement activation
Explanation: ***Type IV hypersensitivity from drug-induced delayed T-cell response***
- **Methotrexate-induced hypersensitivity pneumonitis** manifests as a **Type IV hypersensitivity** reaction, characterized by T-cell mediated inflammation and **non-caseating granulomas**.
- The findings of **CD8+ T lymphocytes** in BAL and multinucleated giant cells on biopsy are classic hallmarks of a **delayed-type hypersensitivity** response.
*Type III hypersensitivity from methotrexate-protein immune complexes*
- **Type III** reactions involve the deposition of **immune complexes** and subsequent **complement activation**, typically resulting in vasculitis rather than granulomatous lung disease.
- While immune complexes can cause lung injury, they do not primarily present with **granulomas** or high levels of **CD8+ T cells** in the alveoli.
*Type I hypersensitivity from methotrexate-induced IgE production*
- **Type I hypersensitivity** is an **IgE-mediated** immediate reaction that results in mast cell degranulation, causing **bronchospasm** or anaphylaxis.
- This reaction occurs within minutes, whereas the patient's **progressive dyspnea** and chronic biopsy findings are inconsistent with an immediate allergic response.
*Type II hypersensitivity from anti-basement membrane antibodies*
- **Type II hypersensitivity** involves **cytotoxic antibodies** targeting specific cell surfaces or tissues, such as the **anti-GBM antibodies** seen in Goodpasture syndrome.
- This mechanism leads to linear immunofluorescence and **alveolar hemorrhage**, which is not supported by the finding of **non-caseating granulomas**.
*Mixed Type II and III hypersensitivity with complement activation*
- A mixed Type II and III response would present with severe **tissue destruction** and systematic low **complement levels**, often seen in systemic lupus erythematosus.
- Such a mechanism does not account for the **T-cell mediated granulomatous** pattern specifically associated with methotrexate lung toxicity.
Question 10: A 28-year-old woman presents with a butterfly rash, photosensitivity, and joint pain for 3 months. Laboratory studies show positive ANA (1:640), anti-dsDNA antibodies, and decreased C3 and C4 complement levels. Urinalysis reveals proteinuria (3+) and RBC casts. Apply your understanding of immune complex-mediated disease to determine the primary pathologic mechanism.
A. Type III hypersensitivity with immune complex deposition (Correct Answer)
B. Type I hypersensitivity with IgE-mediated mast cell degranulation
C. Type IV hypersensitivity with delayed T-cell mediated response
D. Type II hypersensitivity with antibody-mediated cytotoxicity
E. Type V hypersensitivity with antibody-mediated receptor stimulation
Explanation: ***Type III hypersensitivity with immune complex deposition***
- Systemic Lupus Erythematosus (SLE) involves the formation of **circulating antigen-antibody complexes** that deposit in tissues like the **renal glomeruli**, leading to glomerulonephritis.
- Deposition triggers the **complement cascade**, resulting in decreased **C3 and C4 levels** and the recruitment of neutrophils that cause tissue damage.
*Type I hypersensitivity with IgE-mediated mast cell degranulation*
- This mechanism is associated with **atopic reactions** like asthma, hay fever, and anaphylaxis involving **IgE antibodies**.
- It does not explain the formation of **anti-dsDNA antibodies** or the presence of **RBC casts** and proteinuria seen in this patient.
*Type IV hypersensitivity with delayed T-cell mediated response*
- This is a cell-mediated response involving **sensitized T-lymphocytes**, which is characteristic of conditions like the **tuberculin skin test** or contact dermatitis.
- While T-cells play a role in the pathogenesis of SLE, the acute presentation of **malar rash** and **lupus nephritis** is primarily driven by immune complex deposition.
*Type II hypersensitivity with antibody-mediated cytotoxicity*
- While SLE can have Type II components (e.g., **autoimmune hemolytic anemia**), it involves antibodies binding directly to **cell-surface antigens**.
- The systemic features and renal involvement with **low complement** are hallmark indicators of **Type III** rather than Type II hypersensitivity.
*Type V hypersensitivity with antibody-mediated receptor stimulation*
- This involves antibodies that mimic or block **hormone-receptor** activity, such as in **Graves' disease** or **Myasthenia Gravis**.
- It is not a mechanism for multi-system inflammatory damage or **glomerular injury** characterized by proteinuria and casts.
