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I've read that when AIDS symptoms show up, life expectancy is about two years if no treatment is applied. On the other hand, I've also read that people having an organ transplant need to take immune system suppressors for the rest of their lives so that their body does not attack the transplanted organ. So, if I understand well, it is a sort of induced AIDS. I do not understand then how it is possible for people with an organ transplant to live more than about two years.

Can someone explain with details the differences between the suppression of the immune system by medicine for organ transplants and the one due to AIDS?

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Patients receiving immunosuppressive drugs do fall into the same category as patients with HIV-related acquired immunodeficiency - that is, they are all immunocompromised and face similar risks of infection, cancer, etc. The difference emerges over time: when the immunosuppression is controlled and monitored, a patient can retain some immune function, and a physician can help a patient adjust and rapidly respond to opportunistic infections. Drugs can be given in calculated doses; viral infections which disable the body's ability to stop viral infections typically only get worse. To elaborate:

HIV causes immunodeficiency by killing T cells. Firstly, it preferentially infects CD4+ T cells. This process can trigger an inflammatory response and (1) kill the infected T cell, as well as (2) bystander cells. Once the virus has successfully infected a CD4+ T cell, it can happily produce more viruses without killing the cell, which is how the infection can spread - or it can (3) rupture the cell membrane and kill the T cell. Finally, CD8+ T cells can notice that CD4+ T cells are getting infected by a virus and start (4) killing CD4+ T cells.

Immunosuppression after organ transplant can be more specific. It is typically achieved via a combination of drugs which can (1) reduce immune cell activation, (2) slow down immune cell proliferation, or (3) block the activating receptors on T or B cells. The amount and type of suppressants depend on the phase of treatment: induction of immunosuppression, maintenance, or treatment of transplant rejection.1 These drugs are also often administered alongside preventative antimicrobial drugs.

The immunosuppressants which are administered during the maintenance phase - i.e. for the rest of the patient's life - are often the drugs which dampen immune activation instead of eradicating all T cells, and are given at much lower doses. For example, calcineurin inhibitors block the [calcineurin-NFAT pathway];2 when this pathway is blocked, production of immune activators like IL-2 is reduced, and reduces T cell function.

TL;DR: "induced AIDS" is milder and more controlled than actual AIDS, and doctors work very hard to keep patients within a safe range of immune function/suppression.

PS: Good question! Made me think :) I've written some papers on organ transplantation before so am happy to do some digging to add sources.

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