MLT GlobeImmunology & Serology: Autoimmune & hypersensitivity – page 4
80 Immunology & Serology MCQs on Autoimmune & hypersensitivity with answers and explanations.
A patient with recurrent venous thrombosis has a prolonged aPTT that does not correct on mixing with normal plasma and a positive dilute Russell viper venom time. Why is the aPTT prolonged in vitro?
Lupus anticoagulants bind phospholipid-protein complexes and prolong phospholipid-dependent tests in vitro, yet cause thrombosis in vivo. Factor VIII deficiency would correct on mixing and causes bleeding, not thrombosis.
For laboratory classification of antiphospholipid syndrome, a positive lupus anticoagulant, anticardiolipin or anti-beta-2 glycoprotein I result must be:
Antiphospholipid antibodies can appear transiently with infection or drugs, so persistence is required on two occasions at least 12 weeks apart. A positive ANA is not part of the criteria.
A 55-year-old man has nephrotic syndrome. Serum antibodies to the phospholipase A2 receptor (PLA2R) are positive. The most likely diagnosis is:
Anti-PLA2R antibodies are found in most cases of primary membranous nephropathy and help separate it from secondary forms. Anti-GBM disease is marked by anti-GBM antibodies with linear staining.
A child with suspected celiac disease has selective IgA deficiency. Which serologic test is most appropriate?
IgA-based tests are falsely negative in IgA deficiency, so IgG tests such as IgG anti-DGP (or IgG anti-tTG) are used. IgA anti-endomysial antibody would also be falsely negative.
Repeated subcutaneous injections of an antigen in a sensitised person cause local oedema, haemorrhage and necrosis at the site after 4–8 hours. This is called:
The Arthus reaction is a localised type III reaction: circulating IgG meets injected antigen in the vessel wall, forming complexes that fix complement. Serum sickness is the systemic form of type III disease.
An elderly patient has tense skin blisters. Direct immunofluorescence of perilesional skin shows linear IgG and C3 along the basement membrane zone. The most likely diagnosis is:
Bullous pemphigoid has autoantibodies to hemidesmosome proteins (BP180, BP230), giving linear basement membrane staining. Pemphigus vulgaris shows intercellular 'fishnet' staining from anti-desmoglein antibodies.
A patient has fatigable ptosis and weakness typical of myasthenia gravis, but acetylcholine receptor antibodies are negative. Which antibody should be tested next?
About a third to half of AChR-negative generalised myasthenia gravis cases have anti-MuSK antibodies. Voltage-gated calcium channel antibodies indicate Lambert-Eaton myasthenic syndrome, which often improves with repeated effort.
A patient with systemic lupus erythematosus has an active flare with nephritis. Which complement pattern is most expected?
Immune complexes activate the classical pathway, consuming both C4 and C3; levels often rise again with remission. High levels are seen as acute-phase responses, not in active lupus nephritis.
A young woman has a butterfly facial rash, joint pain, a positive ANA and anti-dsDNA antibodies. The most likely diagnosis is:
Malar rash, arthritis, a positive ANA and anti-dsDNA (highly specific) together strongly point to SLE.
In type III hypersensitivity, tissue damage is caused mainly by:
Immune complexes deposit in vessel walls, glomeruli and joints, activate complement and attract neutrophils, which cause the damage, as in serum sickness and lupus nephritis.
Warm autoimmune hemolytic anemia is usually caused by antibodies of which class?
Warm AIHA is usually caused by IgG autoantibodies reacting best at 37 °C, detected by a positive direct antiglobulin test. Cold agglutinin disease is usually IgM.
A woman has Raynaud phenomenon, calcinosis, sclerodactyly and telangiectasia. Which autoantibody is most likely positive?
These features describe limited cutaneous systemic sclerosis (CREST syndrome), which is strongly associated with anti-centromere antibodies.
During suspected anaphylaxis, which serum marker is most useful to confirm mast cell activation if collected within about 1–3 hours?
Tryptase is released by activated mast cells, peaks around 1 hour and remains elevated for several hours, so it helps confirm anaphylaxis. Total IgE does not rise acutely.
An ANA test is positive at 1:80 with a speckled pattern in an otherwise healthy 60-year-old woman. The best interpretation is:
Low-titre ANA is found in a significant proportion of healthy people, more often in older women. ANA is a sensitive screening test and must be interpreted with clinical findings and specific antibody tests.
A 9-year-old girl has raised transaminases, hypergammaglobulinaemia and negative ANA and ASMA. Which antibody would best support type 2 autoimmune hepatitis?
Type 2 autoimmune hepatitis mainly affects children and is defined by anti-LKM-1 (targeting CYP2D6), usually without ANA or ASMA. Anti-SLA is seen mainly in type 1 disease.
A patient with rapidly progressive diffuse skin thickening develops sudden hypertension and acute kidney injury. Which autoantibody carries the highest risk for this complication?
Anti-RNA polymerase III is linked to diffuse cutaneous systemic sclerosis and scleroderma renal crisis. Anti-centromere is associated with limited cutaneous disease and a lower risk of renal crisis.
A healthy 30-year-old has a HEp-2 ANA with a dense fine speckled nucleoplasm and strongly stained metaphase chromatin. Specific ENA tests are negative. This pattern (anti-DFS70) most likely indicates:
Isolated anti-DFS70 antibodies are common in healthy people and, when alone, make a systemic autoimmune rheumatic disease less likely. SLE-related antibodies such as anti-dsDNA give a homogeneous pattern.
A patient with chronic hepatitis C has purpura, arthralgia and low C4. A serum kept at 37 °C then chilled forms a precipitate containing polyclonal IgG and monoclonal IgM with rheumatoid factor activity. This is:
Type II mixed cryoglobulins contain monoclonal IgM with RF activity bound to polyclonal IgG and are strongly linked to hepatitis C. Type I contains only a monoclonal immunoglobulin, often in myeloma or Waldenström disease.
A smoker with small cell lung cancer has proximal weakness that improves briefly after repeated exercise, dry mouth and reduced reflexes. Which antibody is most likely present?
Lambert-Eaton myasthenic syndrome, often paraneoplastic with small cell lung cancer, is caused by antibodies to presynaptic P/Q-type calcium channels. AChR antibodies define myasthenia gravis, in which weakness worsens with exercise.
A child has chronic mucocutaneous candidiasis, hypoparathyroidism and adrenal insufficiency. A mutation in which gene, needed for thymic expression of tissue antigens, is most likely?
AIRE lets medullary thymic cells express tissue-specific antigens so autoreactive thymocytes are deleted; its loss causes APECED (APS-1). FOXP3 defects cause IPEX through absent regulatory T cells.