An immunosuppressant is a substance that performs immunosuppression of the immune system. They may either be exogenous. as immunosuppressive drugs, or endogenous, as e.g. testosterone.^[1]

After organ transplantation, the body will nearly always reject the new organ(s) due to differences in human leukocyte antigen haplotypes between the donor and recipient. As a result, the immune system detects the new tissue as "hostile", and attempts to remove it by attacking it with recipient leukocytes, resulting in the death of the tissue.

Immunosuppressants are applied as a countermeasure; the side effect is that the body becomes extremely vulnerable to infections and malignancy, much like in an advanced HIV infection.

References

es:Inmunosupresor