immunoglobulin (Ig) superfamily Large and diverse family of proteins that contain immunoglobulin domains or immunoglobulin-like domains. Most are involved in cell–cell interactions or antigen recognition. (Figure 24–48) immunoglobulin domain (Ig domain) Characteristic protein domain of about 100 amino acids that is found in immunoglobulin light and heavy chains. Similar domains, known as immunoglobulin-like (Ig-like) domains, are present in many other proteins, which, together with Igs, constitute the  Ig superfamily. (Figure 24–27) immunogold electron microscopy Method to localize specific macromolecules using a primary antibody that binds to the molecule of interest and is then detected with a secondary antibody to which a colloidal gold particle has been attached. The gold particle is electron-dense and can be seen as a black dot in the electron microscope. (Figure 9–45) immunological memory   Long-lived property of the adaptive immune system that follows a primary immune response to many antigens, such that a subsequent encounter with the same antigen will provoke a more rapid and stronger secondary immune response. (Figure 24–16) immunological self-tolerance   The lack of response of the adaptive immune system to an antigen. Tolerance to self molecules is crucial to avoid autoimmune diseases. (Figure 24–21) immunological synapse The highly organized interface that develops between a T cell and an antigen-presenting cell (APC) or target cell it is in contact with, formed by T-cell receptors binding to antigen–MHC complexes on the APC and cell-adhesion proteins binding to their counterparts on the APCs.
induced fit   A principle for increasing the specificity of substrate recognition by proteins and RNAs. In protein synthesis, a ribosome, or enzyme folds around a codon– anticodon interaction and only when the match is correct is the subsequent reaction allowed to proceed.
induced pluripotent stem cells (iPS cells) Cells that are induced by artificial expression of specific transcription regulators to look and behave like the pluripotent embryonic stem cells that are derived from embryos.
induced regulatory T cell   A regulatory T cell (Treg cell) that develops from naive helper T cells when they are activated in the presence of TGFβ in the absence of IL6.
inflammasome   Intracellular protein complex formed after activation of cytoplasmic NOD-like receptors with adaptor proteins. It contains a caspase enzyme that cleaves pro-inflammatory cytokines from their precursor proteins.
inflammatory response   Local response of a tissue to injury or infection—characterized clinically by redness, swelling, heat, and pain. Caused by invasion of white blood cells, which are attracted by and secrete various cytokines.
inhibitors of apoptosis (IAPs) Intracellular protein inhibitors of apoptosis.
inhibitory G protein (Gi) Trimeric G protein that can regulate ion channels and inhibit the enzyme adenylyl cyclase in the plasma membrane. See also G protein. (Table 15–3, p. 846) inhibitory neurotransmitter   Neurotransmitter that opens transmitter-gated Cl– or K+ channels in the postsynaptic membrane of a nerve or muscle cell and thus tends to inhibit the generation of an action potential.
initial segment   Specialized membrane region at the base of a nerve axon (adjacent to the cell body) that is rich in voltage-gated Na+ channels plus other classes of ion channels that all contribute to the encoding of membrane depolarization into action potential frequency.
initiator caspases Apoptotic caspases that begin the apoptotic process, activating the executioner caspases.