Boldface terms within a definition are also defined in this glossary.

Figures and tables that illustrate defined terms are noted in parentheses.

IgCAMs A family of cell-adhesion molecules that contain multiple immunoglobulin (Ig) domains and mediate Ca²⁺-independent cell-cell interactions. IgCAMs are produced in a variety of tissues and are components of tight junctions. (Figure 20-2)

immunoblotting Technique in which proteins separated by electrophoresis are attached to a nitrocellulose or other membrane, and specific proteins then are detected by use of labeled antibodies; also called Western blotting.

immunoglobulin (Ig) Any of the serum proteins, produced by fully differentiated B cells, that can function as antibodies; also occur in membrane-bound form as part of the B-cell receptor. Immunoglobulins are divided into five main classes (isotypes) that exhibit distinct functional properties. See also antibody. (Figures 23-9 and 23-10)

immunoglobulin (Ig) fold Evolutionarily ancient structural motif found in antibodies, the T-cell receptor, and numerous other eukaryotic proteins not directly involved in antigen-specific recognition; also called Ig domain. (Figure 23-13b)

immunoprecipitation (IP) A technique that uses antibodies to separate a target molecule of interest from other molecules in a complex mixture in solution by cross-linking the target molecule into a large aggregate, resulting in the formation of an insoluble solid (precipitate) that can be easily separated and analyzed.

induced pluripotent stem (iPS) cells A mammalian cell with properties of an embryonic stem cell that is formed from a differentiated cell type by expression of one or more transcription factors or other genes that confer pluripotency.

inflammation Localized response to injury or infection that leads to the activation of immune-system cells and their recruitment to the affected site; marked by the four classical signs of redness, swelling, heat, and pain. (Figure 23-7)

inhibitory synapse A synapse in which the neurotransmitter induces a hyperpolarization of the postsynaptic cell, inhibiting generation of an action potential.

initiation, transcription The process by which an RNA polymerase separates DNA strands and synthesizes the first phosphodiester bond of an RNA chain as templated by the DNA strand that enters the RNA polymerase active site. (Figure 5-11)

initiation factor (IF) One of a group of nonribosomal proteins that promote the proper association of ribosomes and mRNA and are required for initiation of translation (protein synthesis). (Figure 5-24)

initiator A DNA sequence that specifies transcription initiation within the sequence.

inner cell mass (ICM) The part of an early embryo that will form the embryo proper but not the extra-embryonic tissues, including the placenta.

inner mitochondrial membrane The highly invaginated membrane that lies immediately underneath the outer mitochondrial membrane, and that comprises the boundary membrane, cristae, and crista junctions.

inositol 1,4,5-trisphosphate (IP₃) Intracellular second messenger produced by cleavage of the membrane lipid phosphatidylinositol 4,5-bisphosphate in response to stimulation of certain cell-surface receptors. IP₃, which triggers release of Ca²⁺ stored in the endoplasmic reticulum, is one of several biologically active phosphoinositides. (Figure 15-6; Table 15-4)

in situ hybridization Any technique for detecting specific DNA or RNA sequences in cells and tissues by treating samples with single-stranded RNA or DNA probes that hybridize to the sequence of interest. (Figure 6-25)

insulin A protein hormone produced in the β cells of the pancreatic islets that stimulates uptake of glucose into muscle and fat cells; acts with glucagon to help regulate blood glucose levels. Insulin also functions as a growth factor for many cells.

integral membrane protein Any protein that contains one or more hydrophobic segments embedded within the core of the phospholipids bilayer; also called transmembrane protein. (Figure 13-10)

integrins A large family of heterodimeric transmembrane proteins that function as adhesion receptors, promoting cell-matrix adhesion, or as cell-adhesion molecules, promoting cell-cell adhesion. (Table 20-4)

interferons (IFNs) Small group of cytokines that bind to cell-surface receptors on target cells inducing changes in gene expression that lead to an antiviral state or other cellular responses important in immune responses.

interleukins (ILs) Large group of cytokines, some released in response to inflammation, that promote proliferation and functioning of T cells and antibody-producing B cells of the immune system.

intermediate filament Cytoskeletal fiber (10 nm in diameter) formed by polymerization of related, but tissue-specific, subunit proteins, including keratins, lamins, and neurofilaments. (Figure 18-47; Table 18-1)

intermembrane space The mitochondrial compartment between the inner and outer mitochondrial membranes, which is continuous with the spaces inside the cristae.

interneurons Nerves that receive signals from other nerve cells and that in turn transmit signals to other nerve cells.

G-14

interphase Long period of the cell cycle, including the G₁, S, and G₂ phases, between one M (mitotic) phase and the next. (Figures 1-16 and 19-1)

interspersed repeats Sequences from transposons that occur at multiple sites throughout the genomes of multicellular animals and plants. (Figure 8-8; Table 8-1)

intron Part of a primary transcript (or the DNA encoding it) that is removed by splicing during RNA processing and is not included in the mature, functional mRNA, rRNA, or tRNA.

in vitro Referring to experiments or manipulations performed outside a cell (including cell fragments, lysates, or purified molecules) or to cells placed in an artificial environment such as in a petri dish or test tube; literally, in glass.

in vivo Referring to experiments or manipulations performed in the context of an intact organism or intact cell, in contrast to experiments using cell fragments, lysates, or purified molecules; literally, in the living.

ionic interaction A noncovalent interaction between a positively charged ion (cation) and negatively charged ion (anion); commonly called ionic bond.

IP₃ See inositol 1,4,5-trisphosphate.

isoelectric point (pI) The pH of a solution at which a dissolved protein or other potentially charged molecule has a net charge of zero and therefore does not move in an electric field. (Figure 3-39)

isoform One of several forms of the same protein whose amino acid sequences differ slightly and whose general activities are similar. Isoforms may be encoded by different genes or by a single gene whose primary transcript undergoes alternative splicing.

isotonic Referring to a solution whose solute concentration is such that it causes no net movement of water in or out of cells.