anaphylatoxins   Pro-inﬂammatory complement fragments C5a and C3a released by cleavage during complement activation. They are recognized by specific receptors, and recruit ﬂuid and inﬂammatory cells to the site of their release.
anaphylaxis A rapid-onset and systemic allergic reaction to antigen, for example to insect venom injected directly into the bloodstream, or to foods such as peanuts. Severe systemic reactions can be potentially fatal due to circulatory collapse and suffocation from tracheal swelling. It usually results from antigens binding to IgE bound by Fcε receptors on mast cells, leading to systemic release of inﬂammatory mediators.
anchor residues Specific amino acid residues in antigenic peptides that determine peptide binding specificity to MHC class I molecules. Anchor residues for MHC class II molecules exist but are less obvious than for MHC class I.
anergy A state of nonresponsiveness to antigen. People are said to be anergic when they cannot mount delayed-type hypersensitivity reactions to a test antigen, whereas T cells and B cells are said to be anergic when they cannot respond to their specific antigen under optimal conditions of stimulation.
ankylosing spondylitis   Inﬂammatory disease of the spine leading to vertebral fusion strongly associated with HLA-B27.
antibody A protein that binds specifically to a particular substance—called its antigen. Each antibody molecule has a unique structure that enables it to bind specifically to its corresponding antigen, but all antibodies have the same overall structure and are known collectively as immunoglobulins. Antibodies are produced by differentiated B cells (plasma cells) in response to infection or immunization, and bind to and neutralize pathogens or prepare them for uptake and destruction by phagocytes.
antibody combining site See antigen-binding site.
antibody-dependent cell-mediated cytotoxicity (ADCC) The killing of antibody-coated target cells by cells with Fc receptors that recognize the constant region of the bound antibody. Most ADCC is mediated by NK cells that have the Fc receptor FcγRIII on their surface.
antibody-directed enzyme/pro-drug therapy (ADEPT) Treatment in which an antibody is linked to an enzyme that metabolizes a nontoxic pro-drug to the active cytotoxic drug.
antibody repertoire The total variety of antibodies in the body of an individual.
antigen Any molecule that can bind specifically to an antibody or generate peptide fragments that are recognized by a T-cell receptor.
antigen-binding site The site at the tip of each arm of an antibody that makes physical contact with the antigen and binds it noncovalently. The antigen specificity of the site is determined by its shape and the amino acids present.
antigenic determinant That portion of an antigenic molecule that is bound by the antigen-binding site of a given antibody or antigen receptor; it is also known as an epitope.
antigenic drift The process by which inﬂuenza virus varies genetically in minor ways from year to year. Point mutations in viral genes cause small differences in the structure of the viral surface antigens.
antigenic shift A radical change in the surface antigens of inﬂuenza virus, caused by reassortment of their segmented genome with that of another inﬂuenza virus, often from an animal.
antigenic variation Alterations in surface antigens that occur in some pathogens (such as African trypanosomes) from one generation to another, which allows them to evade preexisting antibodies.
antigen presentation The display of antigen on the surface of a cell in the form of peptide fragments bound to MHC molecules. T cells recognize antigen when it is presented in this way.