In medicine, immunodeficiency (or immune deficiency) is a state in which the immune system's ability to fight infectious disease is compromised or entirely absent. Most cases of immunodeficiency are either congenital or acquired. A person who has an immunodeficiency is said to be immunocompromised. An immunocompromised person is very vulnerable to opportunistic infections.

Congenital (or Primary) immune deficiency

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A number of rare diseases feature a heightened susceptibility to infections from childhood onward. Many of these disorders are hereditary and autosomal recessive. Examples include myeloperoxidase deficiency, Wiskott-Aldrich syndrome, chronic granulomatous disease, and severe combined immunodeficiency.

Acquired immune deficiency

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Although elderly people are generally more susceptible to infections, this can be increased in the presence of a number of blood disorders (see hematology); important examples are multiple myeloma and chronic lymphatic leukemia (CLL).

Immune suppression is a common side-effect of many drugs, most importantly chemotherapy. However, Disease-modifying antirheumatic drugs (DMARDs) are used to suppress immune activity in autoimmune disorders to slow progression of this type of disease. Any drug that causes neutropenia and/or agranulocytosis can cause immune suppression and sepsis. Organ transplant recipients are given immunosuppressive drugs to avoid transplant rejection.

Immune deficiency is also the hallmark of Acquired Immunodeficiency Syndrome (AIDS). HIV destroys CD4⁺-T-lymphocytes (Helper T cells), leading to opportunistic infections.

Biochemistry

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In all higher mammals a complex set of biochemicals is present that regulates the immune system. Imbalances of these chemicals by nutritional deficit, disease or infection can compromise the immune system.

Immune system disorders

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