MLT GlobeImmunology & Serology: Immune system principles – page 9
257 Immunology & Serology MCQs on Immune system principles with answers and explanations.
Which is required in the alternative pathway but not in the classical pathway?
Factor B (with factor D and properdin) forms the alternative C3 convertase. C3 and later components are shared.
The alternative complement pathway is triggered by:
Microbial surfaces stabilize C3b, triggering the alternative pathway. Antigen–antibody complexes trigger the classical pathway.
Which step belongs to a normal adaptive immune response rather than allergy?
Normal immune response begins with antigen processing and presentation. The others describe type I hypersensitivity.
Which cell is the central coordinator of adaptive immunity, directing both humoral and cellular responses?
CD4 helper T cells secrete cytokines that activate B cells, cytotoxic T cells and macrophages.
Which innate immune cell is most critical for clearing invasive Candida and Aspergillus infections, as shown by risk in neutropenic patients?
Neutropenia is the major risk factor for invasive candidiasis and aspergillosis because neutrophils kill hyphae and yeasts by oxidative burst. Antibody (B cells) plays a minor role.
Tumour-specific transplantation antigens (TSTAs) are best described as molecules that are:
TSTAs arise from mutated or viral proteins unique to the tumour, so they are absent from normal cells. Oncofetal molecules shared with embryonic tissue are tumour-associated antigens.
Alpha-fetoprotein and CEA are tumour-associated antigens. What property distinguishes such antigens from tumour-specific ones?
Tumour-associated (oncofetal) antigens are normal proteins re-expressed or overexpressed by tumours; they occur in fetal tissue and at low levels in adults, which limits their specificity.
Lymphokine-activated killer (LAK) cells used in cancer immunotherapy are generated mainly by culturing which cells with IL-2?
LAK cells are predominantly NK cells (with some T cells) stimulated by high-dose IL-2, which then lyse tumour cells without MHC restriction.
A tumour avoids destruction by cytotoxic T cells. Which change in the tumour most directly allows this escape?
Cytotoxic T cells need antigen presented on MHC class I. Antigen loss (or MHC I loss) removes the target; increasing either would make killing more likely.
After eye trauma, antibodies to lens proteins appear. These proteins are 'sequestered antigens', meaning they:
Sequestered antigens (lens, sperm, some CNS proteins) are anatomically isolated, so tolerance never develops; release after injury can trigger autoimmunity, e.g. sympathetic ophthalmia.
Natural killer (NK) cells are best identified by which marker combination?
NK cells lack CD3 (no T-cell receptor) but express CD16 and CD56. CD3+ CD56+ cells are NKT cells, a different T-cell subset.
Which statement about MHC class II molecules is correct?
Class II (HLA-DR, -DQ, -DP) is expressed mainly on B cells, macrophages and dendritic cells. Beta-2 microglobulin is part of class I, and HLA-A, -B, -C are class I genes.
Papain digestion of an IgG molecule produces:
Papain cuts above the hinge disulfide bonds, giving two monovalent Fab fragments and one crystallizable Fc fragment. Pepsin cuts below the hinge, giving one divalent F(ab')2.
A 25-year-old has recurrent attacks of non-itchy swelling of the face and bowel wall without urticaria. C4 is low between attacks. Which protein is most likely deficient?
Hereditary angioedema is caused by C1 inhibitor deficiency, leading to uncontrolled C1 activity (low C4) and excess bradykinin. Urticaria is typically absent.
A patient has recurrent Neisseria meningitidis infections. Deficiency of which complement component is most likely?
Deficiency of terminal components (C5–C9) prevents formation of the membrane attack complex and predisposes to recurrent Neisseria infections. C1q and C2 deficiencies are more linked to lupus-like disease.
Which cytokine is the main stimulus for hepatic production of C-reactive protein and other acute-phase proteins?
IL-6 (with IL-1 and TNF-alpha) acts on hepatocytes to increase CRP, fibrinogen and serum amyloid A. IL-4 and IL-13 are Th2 cytokines linked to IgE production.
Class switching of B cells to produce IgE is mainly promoted by which cytokine?
IL-4 (and IL-13) from Th2 cells drives switching to IgE. Interferon-gamma and IL-12 favour Th1 responses and inhibit IgE production.
Interferon-gamma's most important effect in cell-mediated immunity is to:
Interferon-gamma, produced by Th1 cells and NK cells, activates macrophages and increases MHC expression. This is key in defense against intracellular organisms such as mycobacteria.
Toll-like receptor 4 (TLR4) on macrophages recognises which microbial molecule?
TLR4 binds lipopolysaccharide (endotoxin) of Gram-negative bacteria. TLR5 recognises flagellin, TLR3 double-stranded RNA and TLR9 CpG DNA.
Regulatory T cells that maintain self-tolerance are typically characterised by:
Regulatory T cells express CD4, high CD25 (IL-2 receptor alpha) and the transcription factor FOXP3. Loss of FOXP3 causes the severe autoimmune syndrome IPEX.