Antennapedia complex One of two gene clusters in Drosophila that contain Hox genes; genes in the Antennapedia complex control the differences among the thoracic and head segments of the body.
anti-apoptotic Bcl2 family proteins   Proteins (e.g., Bcl2, BclXL) on the cytosolic surface of the outer mitochondrial membrane that bind and inhibit pro-apoptotic Bcl2 family proteins and thereby help prevent inappropriate activation of the intrinsic pathway of apoptosis.
anti-IAP   Produced in response to various apoptotic stimuli and, by binding to IAPs and preventing their binding to a caspase, neutralize the inhibition of apoptosis provided by IAPs.
antibiotic   Substance such as penicillin or streptomycin that is toxic to microorganisms. Often a natural product of a particular microorganism or plant.
antibody   Protein secreted by activated B cells in response to a pathogen or foreign molecule. Binds tightly to the pathogen or foreign molecule, inactivating it or marking it for destruction by phagocytosis or complement-induced lysis. (Figure 24–23) antibody response   Adaptive immune response in which B cells are activated to secrete antibodies that circulate in the bloodstream or enter other body fluids, where they can bind specifically to the foreign antigen that stimulated their production.
anticodon   Sequence of three nucleotides in a transfer RNA (tRNA) molecule that is complementary to a three-nucleotide codon in a messenger RNA (mRNA) molecule.
antigen A molecule that can induce an adaptive immune response or that can bind to an antibody or T cell receptor.
antigen-presenting cell   Cell that displays foreign antigen complexed with an MHC protein on its surface for presentation to T lymphocytes.
antigenic determinant   Specific region of an antigen that binds to an antibody or a complementary receptor on the surface of a B cell (BCR) or T cell (TCR).
antigenic variation Ability to change the antigens displayed on the cell surface; a property of some pathogenic microorganisms that enables them to evade attack by the adaptive immune system.
antiparallel   Describes the relative orientation of the two strands in a DNA double helix or two paired regions of a polypeptide chain; the polarity of one strand is opposite to that of the other.
antiporter   Carrier protein that transports two different ions or small molecules across a membrane in opposite directions, either simultaneously or in sequence. (Figure 11–8)
Apaf1   Adaptor protein of the intrinsic apoptotic pathway; on binding cytochrome c, oligomerizes to form an apoptosome.
apical   Referring to the tip of a cell, a structure, or an organ. The apical surface of an epithelial cell is the exposed free surface, opposite to the basal surface. The basal surface rests on the basal lamina that separates the epithelium from other tissue.
apoptosis   Form of programmed cell death, in which a “suicide” program is activated within an animal cell, leading to rapid cell death mediated by intracellular proteolytic enzymes called caspases.
apoptosome   Heptamer of Apaf1 proteins that forms on activation of the intrinsic apoptotic pathway; it recruits and activates initiator caspases that subsequently activate downstream executioner caspases to induce apoptosis.
aquaporin (water channel) Channel protein embedded in the plasma membrane that greatly increases the cell’s permeability to water, allowing transport of water, but not ions, at a high rate across the membrane.
archaeon (plural arch[a]ea) (archaebacterium) Single-celled organism without a nucleus, superficially similar to bacteria. At a molecular level, more closely related to eukaryotes in genetic machinery than are bacteria. Archaea and bacteria together make up the prokaryotes. (Figure 1–17)