Exam 2: Practice Set 2 Flashcards
Which of the following cells have been implicated in the prevention of autoimmune responses (e.g., inflammatory bowel disease) and in the prevention of some nonself responses? A. Antigen-presenting cells B. Anergized T cells C. CD4+CD25+ Treg cells D. Follicular dendritic cells E. Naïve T cells
C. CD4+CD25+ Treg cells
CD4+CD25+ Treg cells inhibit various responses against self-epitopes as well as some responses against epitopes associated with infectious agents and tumors. Antigen-presenting cells do not have this capacity. Anergized cells are inactive. Follicular dendritic cells are involved in the display of antigen to B cells in the LNs. Naïve T cells require activation before they can begin to carry out any of their effector functions.
Which of the following cells require interaction with both peptide-MHC (signal 1) and a set of costimulatory second signals (signal 2) from an Ag-loaded mature dendritic cell to become activated? A. Anergized T cells B. B cells C. Mast cells D. Naïve T cells E. Natural killer cells
The correct answer is D.
Dendritic cells are professional APCs which control the activation of naïve T cells cells. Anergized T cells remain refractory to subsequent engagement of peptide-MHC and remain quiescent. B cells do not require binding of peptide-MHC for activation. Mast cells become activated and degranulated via the binding of antigen to lgE molecules already affixed to the mast cell surfaces. Natural killer cells do not have receptors for binding peptide-MHC.
Which of the following comparisons between Th1 and Th2 cells is true?
A. Th1 cells produce IL-1 but not IL-2, and Th2 cells produce IL-2 but not IL-1.
B. Th1 cells are class I MHC restricted, and Th2 cells are class II MHC restricted.
C. The chemokine receptors CXCR3 and CCR5 are more highly expressed on Th2 cells than
on Th1 cells.
D. Th2 cells are more likely to bind to E-selectin and P-selectin on endothelial cells than are
Th1 cells.
E. Th1 cells produce IFN-γ but not IL-4, and Th2 cells produce IL-4 but not IFN-γ
The correct answer is E. The signature cytokines of Th1 and Th2 cells are IFN-γ and IL-4, respectively. IL-5 and IL-13 are also very specific for Th2 cells. IL-1 is not typically produced by helper T cells of either subset. IL- 2 is produced by Th1 cells. Both Th1 and Th2 cells are CD4+ helper T cells, and therefore are both restricted to recognizing peptide antigens bound to class II MHC molecules. The trafficking patterns of Th1 and Th2 cells differ, and this is related to differences in the expression of adhesion molecules and chemokine receptors. Th1 cells express abundant functional ligands for E-selectin and P-selectin and the chemokine receptors CXCR3 and CCR5, which bind to various chemokines found at sites of active innate immune responses. Th2 cells bind poorly to endothelial selectins and express less CXCR3 and CCR5.
Which of the following molecules is NOT important in the interaction between a cytolytic T lymphocyte and a target cell? A. B7-1 B. ICAM-1 C. LFA-1 D. T cell receptor E. Class I MHC
The correct answer is A. Although naive CD8+ T cells require second signals, such as B7 costimulation, in order to differentiate into effector cytolytic T lymphocyte (CTL), once differentiated, the CTL can kill a target cell that does not express costimulatory molecules. The CTL only requires one signal for killing of the target cell, which depends on the T cell receptor binding to a peptide-class I MHC complex on the surface of the target cell. Tight adhesion between the CTL and target cell is also required, and this is often mediated by T cell integrin LFA-1 binding to target cell ICAM-1.
A 5-year-old boy has a history of recurrent pneumococcal pneumonia, Pneumocystis carinii pneumonia (PCP), and bacterial ear infections. His maternal uncle and an older brother experienced the same symptoms, but he has an older sister who is healthy. Laboratory studies indicate normal numbers of B cells and T cells, and the serum contains mostly IgM and very little IgG. If genes this patient did have a sister affected with the same condition, which of the following would most likely contain a mutation? A. AID (activation-induced deaminase) B. CD40 C. CD40L D. CD28 E. CTLA-4
The correct answer is B.
If the patient’s sister is also affected with hyper-IgM syndrome, then the inheritance pattern is autosomal recessive. Both the AID and CD40 genes are located on autosomal chromosomes, and mutations in both have been identified as causes of hyper-IgM syndrome. However, only a mutation in CD40 will result in reduced macrophage function and susceptibility to Pneumocystis carinii pneumonia, as is observed in this patient.
Which one of the following statements accurately describes antigen recognition events in a lymph node during a helper T cell–dependent antibody response to a protein antigen?
A. Naive B cells and naive T cells simultaneously recognize the intact protein antigen.
B. Naive B cells recognize intact proteins, generate peptide fragments of these proteins, and present them in complexes with major histocompatibility complex (MHC) molecules
to naive helper T cells.
C. Naive B cells recognize intact proteins, generate peptide fragments of these proteins, and present them in complexes with MHC molecules to differentiated helper T cells.
D. Naive T cells recognize peptides bound to MHC molecules presented by dendritic cells, and naive B cells recognize the intact protein antigen bound to the surface of follicular dendritic cells.
E. Differentiated helper T cells recognize peptides bound to MHC molecules on dendritic cells, and the T cells secrete cytokines that promote antibody production by any nearby B cells that have recognized different protein antigens.
The correct answer is C.
T cells and B cells cannot recognize the same protein antigen molecule simultaneously because T cells only recognize peptide-MHC complexes. B cells bind intact proteins, internalize them via surface Ig, and then present peptide-MHC complexes to helper T cells, not to naive T cells. The helper T cells specific for the peptide-MHC complexes have been differentiated from naive T cells that recognized the same peptide-MHC complexes presented by dendritic cells. Follicular dendritic cells display intact protein antigens to previously activated (but not naive) B cells during the germinal center reaction. Collaboration of T cells and B cells requires direct contact of T cells and B cells specific for the same antigen, even though the antigen recognition events are not simultaneous, because the bidirectional activation requires membrane-bound molecules (i.e., CD40 ligand on the T cells and CD40 on the B cells).
Which of the following mechanisms contributes to the change from B cell production of membrane Ig to secreted Ig?
A. V(D)J recombinase-mediated deletion of the exon encoding the transmembrane domain
B. Alternative processing of primary RNA transcripts to remove the transmembrane domain and include a secretory tail piece
C. Increased vesicular exocytosis of intracellular stores of the secretory form of Ig
D. Switch recombinase-mediated recombination of the heavy chain locus to juxtapose the V(D)J segment with the exon encoding a secretory tail piece
E. Up-regulation of enzymes that proteolytically cleave membrane Ig heavy chains just proximal to the membrane
The correct answer is B.
Primary transcripts of Ig genes include sequences encoding both transmembrane and secretory tail piece domains. Alternative splicing of these transcripts determines which form of Ig is ultimately made. V(D)J recombinases are not involved in modifications of Ig heavy chain expression, and switch recombinases are only involved in changes in DNA related to isotype switching. Membrane Ig is not cleaved to form secretory Ig.
Which one of the following molecules is important for the production of IgE antibodies? A. CD28 B. CD40 C. IFN-γ D. IL-2 E. TGF-β
The correct answer is B.
Cytokines play essential roles in regulating the switch to particular heavy chain isotypes. IFN-γ is important in switching to the IgG isotype, whereas TGF-β is a stimulator of IgA production. The cytokine that promotes IgE production is IL-4, but this is not an answer choice. However, the process of isotype switching in general is known to depend on signaling through CD40L-CD40, and is also dependent on the activity of the enzyme activation-induced deaminase (AID). CD28 is a costimulatory molecule that is important for T cell activation and CD40L up-regulation. However, other costimulatory molecules are also present on T cells that can cause up-regulation of CD40L. IL-2 is a growth factor cytokine for T cells.
Which of the following antigenic structures might activate B cell antibody production without the aid of T cells? A. Lysozyme B. Benzene C. Glucose-6-phosphate D. ABO blood group antigen E. Rh factor antigen
The correct answer is D.
T cell–independent antigens consist of polysaccharides, glycolipids, and nucleic acids with multiple repeated epitopes, so that maximal cross-linking of the B cell receptor is induced, thus bypassing the need for T cell help. Of the answer choices, the best choice is the ABO blood group antigen, because of its polyvalent glycolipid structure. Lysozyme and the Rh factor are protein antigens. Benzene and glucose-6-phosphate are not polyvalent.
The symptoms of Toxic Shock Syndrome result from excessive activation of: A. B-cells B. CD4+ T cells C. CD8+ T cells D. dendritic cells E. plasma cells
The correct answer is B The superantigen exotoxins of Staphylococcus aureus and Streptococcus pyogenes are among the most potent T cell mitogens known. The superantigens able to bind MHC class II and the TCR simultaneously. Toxic shock syndrome is a complication of Staphylococcus aureus and Streptococcus pyogenes infections in which bacterial toxins act as superantigens, activating very large numbers of CD4+ T cells and generating an overwhelming immune-mediated cytokine avalanche that manifests clinically as fever, rash, shock, and rapidly progressive multiple organ failure, often in young, previously healthy patients.
The role of the APC in the immune response is all of the following except:
A. the limited catabolism of polypeptide antigens
B. to allow selective association of MHC gene products and peptides
C. to supply second signals required to fully activate T cells
D. to present nonself-peptides associated with MHC class I molecules to CD4+ T cells
E. to present peptide-MHC complexes to T cells with the appropriate receptor
The correct answer is D The APC presents peptide-MHC class I to CD8+ T cells and peptide-MHC class II to CD4+ T cells. The other statements are all features of an APC such as a dendritic cell.
The DNA for an H chain in a B cell making IgG2 antibody for diphtheria toxoid has the following structure: 5'-V17D5J2---Cγ2-Cγ4-Cε-Cα2-3'. How many individual rearrangements were required to go from the embryonic DNA to this B-cell DNA? A. 1 B. 2 C. 3 D. 4 E. 5
The correct answer is C. Three DNA rearrangements are required. First, D5J2 rearrangement occurs, followed by V17D5J2. This permits synthesis of IgM and IgD molecules using V17D5J2. The third rearrangement is the class switch of V17D5J2CμCδ to V17D5J2Cγ2, leading to the synthesis of IgG2 molecules.
If you had 50V, 20D, and 6 J regions able to code for a heavy chain and 40V and 5 J-region genes able to code for a light chain, you could have a maximum repertoire of:
A. 76 +45 = 121 antibody specificities
B. 76 x 45 = 3420 specificities
C. (40 x 5) + (50 x 20 x 6) = 6200 specificities
D. (40 x 5) x (50 x 20 x 6) = 1,200,000 specificities
E. more than 1,200,000 specificities
The correct answer is E.
While 1,200,000 would be the product of all possible combinations of genes, many more antibody specificities are likely to be generated as a result of junctional diversity at the sites of V, D, and J gene segment joining (caused by imprecise joining, deletions, and nucleotide insertions) and somatic hypermutation.
The ability of a single B cell to express both lgM and IgD molecules on its surface at the same time is made possible by:
A. allelic exclusion
B. isotype switching
C. simultaneous recognition of two distinct antigens
D. alternative RNA splicing
E. use of genes from both parental chromosomes
The correct answer is D
The simultaneous synthesis of lgM and IgD is made possible by the alternative splicing of the primary RNA transcript 5’-VDJ-CμCδ-3’ to give either 5’-VDJ-Cμ-3’ or 5’-VDJ-Cδ-3’.
Which of the following statements concerning the organization of lg genes is correct?
A. V and J regions of embryonic DNA have already undergone a rearrangement.
B. Light-chain genes undergo further rearrangement after surface IgM is expressed.
C. VH gene segments can rearrange with Jκ or Jλ gene segments.
D. The VDJ segments coding for an Ig VH region may associate with different heavy-chain constant-region genes.
E. After VDJ joining has occurred, a further rearrangement is required to bring the VDJ unit
next to the Cμ gene.
The correct answer is D The association of VDJ segments coding for an Ig VH region with different heavy-chain constant- region genes is the basis of isotype or class switching.
