An immunosuppressant is any substance that performs immunosuppression of the immune system. They may be either exogenous, as immunosuppressive drugs, or endogenous, as ,e. g., testosterone. When the immune system function is suppressed, there is an increased susceptibility to infectious diseases and cancers.
The term immunotoxin is also sometimes used (incorrectly) to label undesirable immunosuppressants, such as various pollutants. Polychlorinated biphenyls (PCB) and the herbicide DDT are immunosuppressants.
Immunosuppressants may be prescribed when a normal immune response is undesirable, such as in autoimmune diseases.
After an 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 more vulnerable to infections and malignancy, much like in an advanced HIV infection.
BK virus (more dangerous to the immunosuppressed)
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