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?

Question

A.  MHC genes are unique to humans. Without them, the T cells have no way to distinguish self and non-self.
B.  Marsupials lack an adaptive immune system.
C.  Low genetic diversity makes the MHC genes invariant, so the "transplanted" tissue is accepted as self.
D.  Tumors are never targeted by T cells, unless they are caused by a viral infection.


Clarify Question
What is the key concept addressed by the question?
What type of thinking is required?

Gather Content
What do you already know about tissue transplants? What other information is related to the question?
 
Choose Answer Given what you now know, what information is most likely to produce the correct answer?
 
Reflect on Process
Did your problem-solving process lead you to the correct answer? If not, where did the process break down or lead you astray? How can you revise your approach to produce a more desirable result?

Answer 1

C.  Low genetic diversity makes the MHC genes invariant, so the "transplanted" tissue is accepted as self.


Clarify Question
What is the key concept addressed by the question?
        · This question is asking about the cause of the lack of tissue rejection in Tasmanian devils.
What type of thinking is required?
        · Evaluate level:
            o This question is asking you to compare and contrast human tissue transplants with tissue transplants in Tasmanian devils, making it an evaluate-level question.
 
Gather Content
What do you already know about tissue transplants? What other information is related to the question?
        · In vertebrates, 4 tissue types give rise to all structures in the body. These are the epithelial, muscle tissue, connective tissue, and nervous tissue.
        · Tissues are arranged into organs.
        · An organ in one individual is made of the same type of tissue in another individual. For instance, heart tissue includes cardiac muscle in all individuals.
        · In addition to the foundational tissues that do not differ between individuals, within a species, there are molecular markers that include species, individual and tissue-level identity.
        · The immune system of one individual recognizes self-markers and does not attack tissue with those markers.
        · Anything that lack self-markers is attacked by the immune system.
        · Within small groups of individuals, such as endangered species, genetic variability may be quite low.
 
Choose Answer Given what you now know, what information is most likely to produce the correct answer?
        · If non-self markers are encountered, the immune system attacks the foreign tissue, which causes transplant rejection.
        · Other organisms have adaptive immunity, especially mammals such as marsupials.
        · T cells do in fact target tumors, so this answer can be disregarded.
        · If there is insufficient genetic variability, it is possible that tissue markers do not differ between individuals. This would result in lack of transplant rejection.
 
Reflect on Process
Did your problem-solving process lead you to the correct answer? If not, where did the process break down or lead you astray? How can you revise your approach to produce a more desirable result?
        · Evaluate level:
            o Answering this question correctly depended on your ability to weigh and judge, or evaluate, tissue transplants in abundant versus endangered species. If you got an incorrect answer, did you remember that transplant rejection occurs because of a lack of similarity in tissue markers, or that endangered species may have little genetic variability, including in the genetic variability that leads to different tissue markers? Did you have trouble weighing the merits of tissue markers to determine the correct answer?