Immune System Diseases

Immune system diseases arise when immune defenses are deficient, misdirected against the body, or excessively reactive to otherwise harmless substances.

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Immunodeficiency: A condition in which one or more parts of the immune system are absent or function inadequately. This category covers diseases caused by weakened or missing immune defenses.
Primary immunodeficiency: An inherited disorder caused by defects in immune-system development or function. These diseases arise from genetic defects rather than an external cause.
Complete blood count: A blood test that measures blood-cell counts, including white blood cells and their major types. Abnormal white-cell counts can provide an initial clue to immune dysfunction.
Infectious disease: A disease caused by infection with a pathogen such as a bacterium, virus, fungus, or parasite. Infections can result from immune deficiency, but are not themselves necessarily immune disorders.
Opportunistic infection: An infection that takes advantage of weakened host defenses to cause disease. These infections are a major consequence of significant immunodeficiency.
Autoimmunity: An immune response directed against the body's own molecules, cells, or tissues. Self-directed immunity defines a major group of immune system diseases.
Secondary immunodeficiency: Impaired immune function acquired through causes such as infection, malnutrition, medication, or cancer. Acquired causes can weaken defenses without an inherited immune defect.
Antinuclear antibody test: A blood test that detects antibodies binding to components of cell nuclei. It can support evaluation for systemic autoimmune diseases, though it is not diagnostic alone.
Cancer: A group of diseases in which abnormal cells grow and spread without normal control. Cancer can impair immunity or involve immune surveillance, but is not defined by immune dysfunction.
Organ damage: Structural or functional injury to an organ caused by disease or external insult. Persistent immune attack can damage organs such as joints, kidneys, or lungs.