IRF9 A member of the IRF family of transcription factors that interacts with activated STAT1 and STAT2 to form the complex called ISGF3, which induces transcription of many ISGs.
IRGM3 A protein that functions in the maintenance and storage of neutral lipid droplets in many types of cells in association with adipose differentiation related protein.
irradiation-sensitive SCID (IR-SCID) A type of severe combined immunodeficiency due to mutations in DNA repair proteins, such as Artemis, that causes abnormal sensitivity to ionizing radiation and defects in V(D)J recombination.
ISGF3 See IRF9.
isoforms   Different forms of the same protein, for example the different forms encoded by different alleles of the same gene.
isolated lymphoid follicles (ILF) A type of organized lymphoid tissue in the gut wall that is composed mainly of B cells.
isolation membrane See phagophore.
isotype The designation of an immunoglobulin chain in respect of the type of constant region it has. Light chains can be of either κ or λ isotype. Heavy chains can be of μ, δ, γ, α, or ε isotype. The different heavy-chain isotypes have different effector functions and determine the class and functional properties of antibodies (IgM, IgD, IgG, IgA, and IgE, respectively).
isotype switching See class switching.
isotypic exclusion Describes the use of one or other of the light-chain isotypes, κ or λ, by a given B cell or antibody.
JAK inhibitors (Jakinibs) Small molecule kinase inhibitors with relative selectivity for one or more of the JAK kinases.
Janus kinase (JAK) family   Enzymes of the JAK–STAT intracellular signaling pathways that link many cytokine receptors with gene transcription in the nucleus. The kinases phosphorylate STAT proteins in the cytosol, which then move to the nucleus and activate a variety of genes.
J chain Small polypeptide chain made by B cells that attaches to polymeric immunoglobulins IgM and IgA by disulfide bonds, and is essential for formation of the binding site for the polymeric immunoglobulin receptor.
JNK See Jun kinase.
Job’s syndrome See hyper IgE syndrome.
joining gene segment, J gene segment Short DNA sequences that encode the J regions of immunoglobulin and T-cell receptor variable domains. In a rearranged light-chain, TCRα, or TCRγ genes, the J gene segment is joined to a V gene segment. In a rearranged heavy-chain, TCRβ, or TCRδ locus, a J gene segment is joined to a D gene segment.
Jun kinase A protein kinase that phosphorylates the transcription factor c-Jun, enabling it to bind to c-Fos to form the AP-1 transcription factor.
junctional diversity The variability in sequence present in antigen-specific receptors that is created during the process of joining V, D, and J gene segments and which is due to imprecise joining and insertion of nontemplated nucleotides at the joins between gene segments.
κ chain One of the two classes or isotypes of immunoglobulin light chains.
K63-linkages   In polyubiquitin chains, the covalent ligation of lysine 63 amino group of one ubiquitin protein with the carboxy terminus of a second ubiquitin. This type of linkage is most associated with activation of signaling by formation of a scaffold recognized by signaling adaptors such as TAB1/2.
killer cell immunoglobulin-like receptors (KIRs) Large family of receptors present on NK cells, through which the cells’ cytotoxic activity is controlled. The family contains both activating and inhibitory receptors.