Hey all, I have an immunology question on my homework that is bugging me. I\'m n
ID: 63952 • Letter: H
Question
Hey all,
I have an immunology question on my homework that is bugging me. I'm not sure I understand the concept of "isografts succeed, allografts fail" rule in general. Any help would be greatly appreciated. Thanks!
Background information:
Dr. Triplett did a transplantation experiment: He removed the pituitary (hypophysectomy) from developing tadpoles. He "parked" these pituitaries in other frogs while the hypophysectomized tadpoles developed into adults. Finally, he transplanted the parked pituitaries back into the original (now adult) frogs. The success of this graft was easily measured: because the pituitary makes melanocyte stimulating hormone (MSH), hypophosectomized frogs are white. Thus, if the pituitary graft succeeded, the frog turned brown again and stayed that way. Indeed, all the frogs turned brown a day after their graft. However, within two weeks, all the frogs turned white again, indicating complete rejection. [Assume that the "parking" step did not change the pituitary in any way - Dr. Triplett controlled for this possibility in other experiments not described here].
Provide an explanation for the following questions:
1. Inasmuch as these grafts were the frogs' own pituitaries, does this break the "isografts succeed, allografts fail" rule?
2. What does this experiment tell you about the underlying mechanism of that last property of the immune system - non-responsiveness to "self?"
Explanation / Answer
1. Phenomena associated with the acceptance and rejection of tissue transplants has led to the establishment of two principles. First, rejection depends on the genetic disparity of donor and host. Second, the processes involved in rejection are immunological in nature.
An isogenic graft or isograft is a graft between genetically identical individuals. Typically, isografts are either grafts between animals of a single highly inbred strain, between the F1 hybrids produced by crossing inbred strains, or between identical twins.
An allogeneic graft or allograft is a graft between genetically disparate individuals of the same species. More specifically, it is a graft in which the grafted tissue carries a histocompatibility allele or alleles, and hence presumably an alloantigen or alloantigens, foreign to the recipient.
So, in the case of tadpole the isografts succeed, allografts fail rule is not broken because the pituitary is transplanted from a tadpole into a new frog which is genetically different from the tadpole, but it is within the same species. When the pituitary is retransplanted into the developed tadpole, it is rejected as it is considered as an alloantigen in the developed tadpole.
2. Immunological tolerance is a state of unresponsiveness that is specific for a particular antigen; it is induced by prior exposure to that antigen. The most important aspect of tolerance, however, is self-tolerance, which prevents the body from mounting an immune attack against its own tissues. There is potential for such attack because the immune system randomly generates a vast diversity of antigen-specific receptors therefore, must be eliminated, either functionally or physically. Self-reactivity is prevented by processes that occur during development, rather than being genetically preprogrammed.
When the tadpoles are developing the pituitary has been removed due to which they have not undergone the thymic selection where the self-antigens are presented to the T cell clones and self-tolerance is achieved or also called as nonresponsiveness to self. When these pituitaries are retransplanted into the developed tadpoles, the rejection of this transplantation is due to pituitary being foreign to the developed tadpole as it has not been exposed to the thymic selection process.