interferon stimulated genes (ISGs) A category of gene induced by interferons, which include many that promote innate defense against pathogens, such as oligoadenylate synthetase, PKR, and the Mx, IFITs, and IFITM proteins.
interferon-α receptor (IFNAR) This receptor recognizes IFN-α and IFN-β to activate STAT1 and STAT2 and induce expression of many ISGs.
interferon-induced transmembrane protein (IFITM) See IFITM.
interferon-producing cells (IPCs) See plasmacytoid dendritic cells.
interferons (IFNs) Several related families of cytokines originally named for their interference of viral replication. IFN-α and IFN-β are antiviral in their effects; IFN-γ has other roles in the immune system.
intergenic control regions Sites in non-coding regions of genes that control their expression and rearrangement by interactions with transcription factors and chromatin-modifying proteins.
interleukin (IL) A generic name for cytokines produced by leukocytes. The more general term cytokine is used in this book, but the term interleukin is used in the naming of specific cytokines such as IL-2. Some key interleukins are listed in the glossary under their abbreviated names, for example IL-1β and IL-2. Cytokines are listed in Appendix III.
intraepithelial lymphocytes (IELs)   Lymphocytes present in the epithelium of mucosal surfaces such as the gut. They are predominantly T cells, and in the gut are predominantly CD8 T cells.
intrathymic dendritic cells See dendritic cells.
intrinsic pathway of apoptosis Signaling pathway that mediates apoptosis in response to noxious stimuli including UV irradiation, chemotherapeutic drugs, starvation, or lack of the growth factors required for survival. It is initiated by mitochondrial damage. Also called the mitochondrial pathway of apoptosis.
invariant chain (Ii, CD74) A polypeptide that binds in the peptide-binding cleft of newly synthesized MHC class II proteins in the endoplasmic reticulum and blocks other peptides from binding there. It is degraded in the endosome, allowing for loading of antigenic peptides there.
invariant NKT cells (iNKT cells) A type of innate-like lymphocyte that carries a T-cell receptor with an invariant α chain and a β chain of limited diversity that recognizes glycolipid antigens presented by CD1 MHC class Ib molecules. This cell type also carries the surface marker NK1.1, which is usually associated with NK cells.
IPEX (immune dysregulation, polyendocrinopathy, enteropathy, X-linked)   Immune dysregulation, polyendocrinopathy, enteropathy, X-linked syndrome. A very rare inherited condition in which CD4 CD25 regulatory T cells are lacking as a result of a mutation in the gene for the transcription factor FoxP3, leading to the development of autoimmunity.
ipilimumab Antibody to human CTLA-4 used to treat melanoma, and first checkpoint blockade immunotherapy.
Ir (immune response) genes An archaic term for genetic polymorphisms controlling the intensity of the immune response to a particular antigen, now known to result from allelic differences in MHC molecules, especially MHC class II molecules, that inﬂuence binding of particular peptides.
IRAK1, IRAK4 Protein kinases that are part of the intracellular signaling pathways leading from TLRs.
IRAK4 deficiency An immunodeficiency characterized by recurrent bacterial infections, caused by inactivating mutations in the IRAK4 gene that result in a block in TLR signaling.