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Micrograph showing an opportunistic infection due to immunosuppression - large (blue) cell below-center-left infected with a polyomavirus. Urine cytology specimen.
Immunosuppression involves an act that reduces the activation or efficacy of the immune system. Some portions of the immune system itself have immuno-suppressive effects on other parts of the immune system, and immunosuppression may occur as an adverse reaction to treatment of other conditions.
In general, deliberately induced immunosuppression is performed to prevent the body from rejecting an organ transplant, treating graft-versus-host disease after a bone marrow transplant, or for the treatment of auto-immune diseases such as rheumatoid arthritis or Crohn's disease. This is typically done using drugs, but may involve surgery (splenectomy), plasmapharesis, or radiation.
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.
Deliberately induced 
Administration of immunosuppressive drugs is the main method of deliberately induced immunosuppression. In optimal circumstances, immunosuppressive drugs are targeted only at any hyperactive component of the immune system, and in ideal circumstances would not cause any significant immunodeficiency. However, in essence, all immunosuppressive drugs have the potential to cause immunodeficiency. Immunodeficiency may manifest as increased susceptibility to opportunistic infections and decreased cancer immunosurveillance.
Cortisone was the first immunosuppressant identified, but its wide range of side-effects limited its use. The more specific azathioprine was identified in 1959, but it was the discovery of cyclosporine in 1970 that allowed for significant expansion of kidney transplantation to less well-matched donor-recipient pairs as well as broad application of liver transplantation, lung transplantation, pancreas transplantation, and heart transplantation.
Throughout its history, radiation therapy has been used to decrease the strength of the immune system. Dr. Joseph Murray of Harvard Medical School and chief plastic surgeon at Children's Hospital Boston from 1972-1985 was awarded the Nobel Prize in Physiology or Medicine in 1990 for his work on immunosuppression.
Non-deliberate immunosuppression 
Non-deliberate immunosuppression can occur in, for example, malnutrition, aging, many types of cancer (such as leukemia, lymphoma, multiple myeloma), and certain chronic infections such as acquired immunodeficiency syndrome (AIDS). The unwanted effect in non-deliberate immunosuppression is immunodeficiency that results in increased susceptibility to pathogens such as bacteria and virus.
Immunodeficiency is also a potential adverse effects of many immunosuppressant drugs. In this sense, the scope of the term immunosuppression in general includes both beneficial and potential adverse effects of decreasing the function of the immune system, whereas the term immunodeficiency in general refers solely to the adverse effect of increased risk for infection.