The transplantation of organs from one individual to another normally involves two major problems: (1) organ rejection is likely unless the transplantation antigens of both individuals are nearly identical, and (2) the introduction of any unmatched transplantation antigens induces the development by the recipient of donor-specific lymphocytes that will produce violent rejection of further transplantations from that donor. However, we have found that among many strains of rats these "normal" rules of transplantation are not obeyed by liver transplants. Not only are liver transplants never rejected, but they even induce a state of donor-specific unresponsiveness in which subsequent transplants of other organs, such as skin, from that donor, are accepted permanently. Our hypothesis is that (1) many strains of rats simply cannot mount a sufficiently vigorous destructive immune-response (using lymphocytes) to outstrip the liver's relatively great capacity to protect itself from immune-response damage and that (2) the systemic unresponsiveness observed is due to concentration of the recipient's donor-specific lymphocytes at the site of the liver transplant.

17. The primary purpose of the passage is to treat the accepted generalizations about organ transplantation in which of the following ways?

(A) Explicate their main features
(B) Suggest an alternative to them
(C) Examine their virtues and limitations
(D) Criticize the major evidence used to support them
(E) Present findings that qualify them



18. It can be inferred from the passage that the author believes that an important difference among strains of rats is the

(A) size of their livers
(B) constitution of their skin
(C) strength of their immune-response reactions
(D) sensitivity of their antigens
(E) adaptability of their lymphocytes



19. According to the hypothesis of the author, after a successful liver transplant, the reason that rats do not reject further transplants of other organs from the same donor is that the

(A) transplantation antigens of the donor and the recipient become matched
(B) lymphocytes of the recipient are weakened by the activity of the transplanted liver
(C) subsequently transplanted organ is able to repair the damage caused by the recipient's immune-response reaction
(D) transplanted liver continues to be the primary locus for the recipient's immune-response reaction
(E) recipient is unable to manufacture the lymphocytes necessary for the immune-response reaction



20. Which of the following new findings about strains of rats that do not normally reject liver transplants, if true, would support the authors' hypothesis?

I. Stomach transplants are accepted by the recipients in all cases.
II. Increasing the strength of the recipient's immune-response reaction can induce liver-transplant rejection.
III. Organs from any other donor can be transplanted without rejection after liver transplantation.
IV. Preventing lymphocytes from being concentrated at the liver transplant produces acceptance of skin transplants.

(A) II only
(B) I and III only
(C) II and IV only
(D) I,II, and III only
(E) I, III, and IV only