acquired immune deficiency syndrome (AIDS) A disease caused by infection with the human immunodeficiency virus (HIV-1). AIDS occurs when an infected patient has lost most of his or her CD4 T cells, so that infections with opportunistic pathogens occur.
activating receptors   On NK cells, a receptor whose stimulation results in activation of the cell’s cytotoxic activity.
activation-induced cell death A process by which autoreactive T cells are induced to die if they complete thymic maturation and migrate to the periphery.
activation-induced cytidine deaminase (AID) Enzyme that initiates somatic hypermutation and isotype switching by deaminating DNA directly at cytosine in immunoglobulin V regions or switch regions. Loss of AID activity in patients leads to loss of both activities, causing hyper IgM and lack of affinity maturation.
activator protein 1 (AP-1) A transcription factor formed as one of the outcomes of intracellular signaling by antigen receptors of lymphocytes.
active immunization Immunization with antigen to provoke adaptive immunity.
acute desensitization An immunotherapeutic technique for rapidly inducing temporary tolerance to, for example, an essential drug such as insulin or penicillin in a person who is allergic to it. Also called rapid desensitization. When performed properly, can produce symptoms of mild to moderate anaphylaxis.
acute phase   In reference to HIV infection, the period that occurs soon after a person becomes infected. It is characterized by an inﬂuenza-like illness, abundant virus in the blood, and a decrease in the number of circulating CD4 T cells.
acute-phase proteins Proteins with innate immune function whose production is increased in the presence of an infection (the acute-phase response). They circulate in the blood and participate in early phases of host defense against infection. An example is mannose-binding lectin.
acute-phase response A change in the proteins present in the blood that occurs during the early phases of an infection. It includes the production of acute-phase proteins, many of which are produced in the liver.
acute rejection The rejection of a tissue or organ graft from a genetically unrelated donor that occurs within 10–13 days of transplantation unless prevented by immunosuppressant treatment.
adaptive immunity Immunity to infection conferred by an adaptive immune response.
adaptors Nonenzymatic proteins that form physical links between members of a signaling pathway, particularly between a receptor and other signaling proteins. They recruit members of the signaling pathway into functional protein complexes.
ADCC See antibody-dependent cell-mediated cytotoxicity.
adenoids Paired mucosa-associated lymphoid tissues located in the nasal cavity.
adenosine deaminase (ADA) deficiency An inherited defect characterized by nonproduction of the enzyme adenosine deaminase, which leads to the accumulation of toxic purine nucleosides and nucleotides in cells, resulting in the death of most developing lymphocytes within the thymus. It is a cause of severe combined immunodeficiency.
adhesins Cell-surface proteins on bacteria that enable them to bind to the surfaces of host cells.
adipose differentiation related protein A protein that functions in the maintenance and storage of neutral lipid droplets in many types of cells.
adjuvant Any substance that enhances the immune response to an antigen with which it is mixed.
afferent lymphatic vessels Vessels of the lymphatic system that drain extracellular ﬂuid from the tissues and carry antigen, macrophages, and dendritic cells from sites of infection to lymph nodes or other peripheral lymphoid organs.
affinity The strength of binding of one molecule to another at a single site, such as the binding of a monovalent Fab fragment of antibody to a monovalent antigen. Cf. avidity.