There is support for both hypotheses and both may

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There is support for both hypotheses and both may contribute to the paradox of thymic selection. Section: Positive and Negative Selection Difficulty: 3 Hint: T cells can distinguish survival signals and negative selection signals. 18. What is currently the best explanation for what determines whether a T cell develops into a CD8 + T cell or a CD4 + T cell? A. CD4/TCR coengagement suppresses CD8 expression, and CD8/TCR coengagement suppresses CD4 expression. B. T cells randomly down-regulate CD4 or CD8, and only cells that still bind the appropriate MHC will get a strong enough signal to survive. C. CD4 or CD8 expression is determined by the strength and duration of the TCR/MHC interaction. D. CD4 or CD8 expression is predestined in the bone marrow. E. Only gd T cells express CD8. Answer: C Section: Lineage Commitment Difficulty: 3 Hint: The kinetic model has the greatest experimental support. 19. Which of the following BEST describes the relationship between natural T REG s and induced T REG s? A. Natural T REG s differentiate into induced T REG s in response to stimulation through the TCR. B. Natural T REG s are derived from memory T cells, and induced T REG s differentiate directly from developing thymocytes. C. Natural T REG s develop in the thymus, and induced T REG s develop in the periphery.
D. Natural T REG s have less diverse TCRs than induced T REG s. E. None of the above Answer: C Section: Exit from the Thymus and Final Maturation Difficulty: 2 Hint: Some autoreactive cells in the thymus gain the ability to suppress other autoreactive cells. 20. Which transcription factor is characteristic of T REG cells? A. AP-1 B. NFAT C. NF k B D. OCT-1 E. FOXP3 Answer: E Section: Exit from the Thymus and Final Maturation Difficulty: 1 Hint: This transcription factor is not characteristic of T-cell activation. 21. T REG s have been shown to be protective against which of the following conditions? A. Inflammatory bowel disease B. HIV disease C. Influenza infection D. MRSA E. Malaria Answer: A Section: Exit from the Thymus and Final Maturation Difficulty: 1 Hint: T REG s can develop from autoreactive cells. 22. Describe the cellular changes that occur during apoptosis. Answer: Cells decrease in cell volume, the membrane blebs, chromatin is condensed, and DNA is degraded into fragments. Section: Apoptosis Difficulty: 3 Hint: Apoptosis is programmed cell death. 23. Which of the following are characteristics of apoptosis? A. Membrane blebbing B. Swelling of the cytoplasm C. Increased metabolism
D. Release of cellular contents into the extracellular space E. All of the above Answer: A Section: Apoptosis Difficulty: 2 Hint: Apoptosis occurs naturally and does not necessarily lead to inflammation.

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