MLT GlobeImmunology & Serology: Immune system principles – page 12
257 Immunology & Serology MCQs on Immune system principles with answers and explanations.
Activated T cells that are repeatedly stimulated can be removed by activation-induced cell death. This is mainly mediated by:
Fas–FasL interaction triggers apoptosis of chronically activated lymphocytes, a peripheral tolerance mechanism; Fas defects cause autoimmune lymphoproliferative syndrome. CD28–B7 is a costimulatory, activating signal.
Plain Haemophilus influenzae type b polysaccharide vaccine works poorly in infants, but the conjugate vaccine works well because protein conjugation:
Polysaccharides are T-independent antigens that young infants respond to poorly. Linking them to a protein carrier lets helper T cells help polysaccharide-specific B cells, producing IgG and memory.
A healthcare worker completed hepatitis B vaccination and has never been infected. Which serological profile is expected?
The recombinant vaccine contains only HBsAg, so it produces anti-HBs alone. Anti-HBc appears only after natural infection.
An infant is diagnosed with severe combined immunodeficiency. Which vaccine must be avoided?
BCG is a live attenuated vaccine and can cause disseminated infection in severe immunodeficiency. Toxoid, inactivated and recombinant vaccines cannot replicate, although responses may be poor.
Thymus-independent antigens, such as repeating bacterial polysaccharides, typically induce:
These antigens cross-link many B-cell receptors without T-cell help, so the response is mostly IgM, with weak affinity maturation and little memory. Strong IgG memory requires T-cell help.
Which weak attractive force arises from temporary fluctuations in electron clouds and works only when antigen and antibody are very close together?
Van der Waals forces come from transient dipoles between electron clouds and act only over very short distances, so a close fit is essential. Ionic bonds need oppositely charged groups.
An antibody binds a native protein but not the same protein after denaturation. The epitope it recognises is most likely:
Conformational epitopes are formed by amino acids brought together by folding, and they are lost when the protein unfolds. Linear epitopes are continuous sequences often still recognised after denaturation.
Which cells are the main source of tumour necrosis factor-alpha (TNF-α) when bacterial endotoxin (LPS) enters the blood?
LPS binds TLR4 on macrophages and monocytes, which then release TNF-α, IL-1 and IL-6, driving fever and septic shock.
A patient has repeated severe pyogenic bacterial infections. Deficiency of which complement component would cause the most serious problems with opsonisation?
C3 is central to all complement pathways, and C3b is the main complement opsonin, so C3 deficiency causes severe recurrent pyogenic infections. C5–C9 deficiency mainly predisposes to Neisseria infections.
Which is an example of artificially acquired passive immunity?
Giving ready-made antibody by injection is artificial passive immunity. Vaccination is artificial active, recovery from infection is natural active, and placental IgG is natural passive.
A patient whose spleen was removed is at particular risk of severe infection with:
The spleen is important for clearing encapsulated bacteria and making antibodies to polysaccharides. Asplenic patients can develop overwhelming pneumococcal, meningococcal and Hib sepsis.
To identify an immunoglobulin's isotype by sequencing, which part of the molecule is most informative?
Isotype is defined by the heavy-chain constant region, toward the C-terminus. Variable regions at the N-terminus determine specificity.
Regarding experimental tolerance induction, which statement is accurate?
T cells are tolerised at lower antigen doses and remain tolerant longer than B cells; both can be tolerant. Simple, soluble, non-aggregated antigens induce tolerance more readily.
Which IgG subclass is least able to activate complement by the classical pathway?
IgG3 and IgG1 activate complement strongly, IgG2 weakly, and IgG4 essentially not at all. IgG4 is also known for Fab-arm exchange.
Antibody specificities that differ because of unique variable-region structures of a particular antibody are called:
Idiotypes are unique determinants in the variable region of a given antibody. Isotypes are class differences (e.g. IgG vs IgM); allotypes are inherited variants between individuals of the same species.
A total hemolytic complement (CH50) result is zero, while the alternative pathway (AH50) activity is normal. The defect most likely lies in:
CH50 tests the classical and terminal pathway; AH50 tests the alternative and terminal pathway. Normal AH50 means C3 and C5–C9 work, so the defect is in C1, C4 or C2.
Th17 cells mainly protect against extracellular bacteria and fungi by producing IL-17, which recruits:
IL-17 induces chemokines that recruit neutrophils to mucosal and skin sites. Th2 cells, not Th17, drive eosinophil responses through IL-5.
A kappa:lambda free light chain ratio in serum is used mainly to detect:
Each B-cell clone makes only one light chain type. A clonal plasma cell disorder produces excess of one type, giving an abnormal kappa:lambda ratio.
Naive CD4 T cells differentiate into Th17 cells mainly in the presence of which cytokine combination?
TGF-β with IL-6 (maintained by IL-23) induces the transcription factor RORγt and the Th17 programme. TGF-β without IL-6 favours regulatory T cells instead.
An infant has severe combined immunodeficiency with absent T cells but normal B-cell numbers. A defect in the receptor for which cytokine, essential for early T-cell development, is a known cause?
IL-7 signalling through IL-7Rα is required for thymocyte development; its loss causes T-negative, B-positive, NK-positive SCID. IL-5 acts on eosinophils.