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Monday 16 March 2009

Different types of immunosuppression affect different cells of the immune system. The opportunistic infections that an immunosuppressed person contracts depend on the types of immune effector mechanisms that are not working correctly. Immunodeficiencies may be genetic (primary) or acquired (secondary).

Patients with antibody deficiency, as in X-linked agammaglobulinemia, contract severe bacterial infections, including S. pneumoniae, H. influenzae, and S. aureus, as well as a few viral infections (rotavirus and enteroviruses).

Patients with deficiencies in complement proteins are particularly susceptible to bacterial infections such as S. pneumoniae, H. influenzae, and N. meningitidis.

Some children have deficiencies in neutrophil function, leading to increased infections with S. aureus as well as some Gram-negative bacteria and fungi.

Diseases of organ systems other than the immune system can also make patients susceptible to specific microorganisms. People with cystic fibrosis commonly get respiratory infections with Pseudomonas aeruginosa, S. aureus, and Burkholdaria cepacia.

The lack of splenic function in individuals with sickle-cell disease makes them susceptible to infection with encapsulated bacteria such as S. pneumoniae, which are normally opsonized and phagocytosed by splenic macrophages.

Acquired immunodeficiencies have a variety of causes, the most important being infection with the human immunodeficiency virus (HIV), which causes AIDS. HIV infects and eventually kills CD4+ helper T lymphocytes.

This leads to profound immunosuppression and a multitude of infections. While most organisms that infect people with AIDS were common pathogens before the era of HIV, some were uncommon infections before HIV (cryptococcus, pneumocystis), and one, Kaposi sarcoma herpesvirus (KSHV), also called human herpesvirus-8 (HHV-8), was discovered as a result of research in HIV patients.

Diseases that impair production of leukocytes, such as leukemia, which fills the bone marrow with cancerous cells, make patients vulnerable to opportunistic infections. Burns destroy skin, removing this barrier to microbes, allowing infection with pathogens such as P. aeruginosa.

Iatrogenic causes of immunosuppression include immunosuppressive drugs used to treat patients with autoimmune diseases and organ transplant recipients as well as drugs used to treat cancer. Finally, malnutrition may impair the immune response.

Therapy to prevent organ transplant rejection leads to severe immunosuppression, making transplant recipients very susceptible to infectious diseases.

Patients receiving bone marrow transplants are profoundly immunosuppressed during the time that the donated bone marrow is engrafting, and became susceptible to infection with almost any organism, including environmental organisms that seldom cause disease in healthy people (e.g., Aspergillus species and Pseudomonas species that are common in water).