T cells binding to MHC-peptide complexes are responsible for acceptance or rejection of transplanted organs. To facilitate acceptance of the transplant, the ideal donor will have a close genetic relationship with the recipient-even then, immune suppression drugs are often necessary. In a strange variation of this phenomenon, the endangered Tasmanian devil of Australia suffers from a devastating disease in which tumorous tissue is transmitted from animal to animal. What is the likely explanation for the lack of rejection of this transmissible tumor?
A. Low genetic diversity makes the MHC genes invariant, so the "transplanted" tissue is accepted as self.
B. Marsupials lack an adaptive immune system.
C. MHC genes are unique to humans. Without them, the T cells have no way to distinguish self and non-self.
D. Tumors are never targeted by T cells, unless they are caused by a viral infection.
Answer: A
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A. As we learn more about the ecology of the human microbiome, we have learned that these organisms are dependent on the nutrients that we provide for their survival. B. As we learn more about the ecology of the human microbiome, we recognize our dependence on these organisms for immunity, vitamin production, and other essential processes. C. As we learn more about the makeup of the human digestive tract, we realize that some microorganisms are potentially pathogenic while others are not. D. All of the choices are correct.
Transcription starts at a region of DNA called a(n) _____
a. sequencer b. promoter c. activator d. terminator e. transcriber
Which are low-growing, nonvascular plants that have flagellated sperm and disperse by producing spores?
a. mosses b. ferns c. liverworts d. hornworts e. lichen
An echinoderm characteristic is the exoskeleton,
____________________ Indicate whether the statement is true or false.