Artemis An endonuclease involved in the gene rearrangements that generate functional immunoglobulin and T-cell receptor genes.
Arthus reaction A local skin reaction that occurs when a sensitized individual with IgG antibodies against a particular antigen is challenged by injection of the antigen into the dermis. Immune complexes of the antigen with IgG antibodies in the extracellular spaces in the dermis activate complement and phagocytic cells to produce a local inﬂammatory response.
aryl hydrocarbon receptor (AhR) A basic helix-loop-helix transcription factor that is activated by various aromatic ligands including, famously, dioxin. It functions in the normal activity of several types of immune cells including some ILCs and IELs.
ASC (PYCARD) An adaptor protein containing pyrin and CARD domains involved in activating caspase 1 in the inﬂammasome.
asymptomatic phase   In reference to HIV infection, period in which the infection is being partly held in check and no symptoms occur; it may last for many years.
ataxia telangiectasia (ATM) A disease characterized by a staggering gait and multiple disorganized blood vessels, and often accompanied by clinical immunodeficiency. It is caused by defects in the ATM protein, which is involved in DNA repair pathways that are also used in V(D)J recombination and class-switch recombination.
atopic march The clinical observation that it is common for children with atopic eczema to later develop allergic rhinitis and/or asthma.
atopy A genetically based increased tendency to produce IgE-mediated allergic reactions against innocuous substances.
ATP-binding cassette (ABC) A large family of proteins containing a particular domain for nucleotide-binding that includes many transporters, such as TAP1 and TAP2, but also various NOD members.
attenuation The process by which human or animal pathogens are modified by growth in culture so that they can grow in their host and induce immunity without producing serious clinical disease.
atypical hemolytic uremic syndrome A condition characterized by damage to platelets and red blood cells and inﬂammation of the kidneys that is caused by uncontrolled complement activation in individuals with inherited deficiencies in complement regulatory proteins.
autoantibodies Antibodies specific for self antigens.
autoantigens A self antigen to which the immune system makes a response.
autocrine Describes a cytokine or other biologically active molecule acting on the cell that produces it.
autograft A graft of tissue from one site to another on the same individual.
autoimmune disease Disease in which the pathology is caused by adaptive immune responses to self antigens.
autoimmune hemolytic anemia A pathological condition with low levels of red blood cells (anemia), which is caused by autoantibodies that bind red blood cell surface antigens and target the red blood cell for destruction.
autoimmune lymphoproliferative syndrome (ALPS) An inherited syndrome in which a defect in the Fas gene leads to a failure in normal apoptosis, causing unregulated immune responses, including autoimmune responses.
autoimmune polyendocrinopathy–candidiasis–ectodermal dystrophy (APECED) A disease characterized by a loss of tolerance to self antigens, caused by a breakdown of negative selection in the thymus. It is due to defects in the gene AIRE, which encodes a transcriptional regulatory protein that enables many self antigens to be expressed by thymic medullary epithelial cells. Also called autoimmune polyglandular syndrome type I.
autoimmune thrombocytopenic purpura An autoimmune disease in which antibodies against platelets are made. Antibody binding to platelets causes them to be taken up by cells with Fc receptors and complement receptors, resulting in a decrease in platelet count that leads to purpura (bleeding).
autoimmunity Adaptive immunity specific for self antigens.