IL-21 A cytokine produced by T cells (e.g.,TFH cells) that activates STAT3 and promotes survival and proliferation, particularly germinal center B cells.
ILC1 A subset of innate lymphoid cells characterized by production of IFN-γ.
ILCs (innate lymphoid cells) These are a class of innate immune cells having overlapping characteristics with T cells but lacking an antigen receptor. They arise in several groups, ILC1, ILC2, ILC3, and NK cells, which exhibit properties roughly similar to TH1, TH2, TH17, and CD8 T cells.
Imd (immunodeficiency) signaling pathway A defense against Gram-negative bacteria in insects that results in the production of antimicrobial peptides such as diptericin, attacin, and cecropin.
imiquimod Drug (aldara) approved for treatment of basal cell carcinoma, genital warts, and actinic keratoses known to activate TLR-7, although not approved as an adjuvant for vaccines.
immature B cells B cells that have rearranged a heavyand a light-chain V-region gene and express surface IgM, but have not yet matured sufficiently to express surface IgD as well.
immediate hypersensitivity reactions Allergic reactions that occur within seconds to minutes of encounter with antigen, caused largely by activation of mast cells or basophils.
immune complexes Complexes formed by the binding of antibody to its cognate antigen. Activated complement proteins, especially C3b, are often bound in immune complexes. Large immune complexes form when sufficient antibody is available to cross-link multivalent antigen; these are cleared by cells of the reticuloendothelial system that bear Fc receptors and complement receptors. Small, soluble immune complexes form when antigen is in excess; these can be deposited in small blood vessels and damage them.
immune evasion Mechanisms used by pathogens to avoid detection and/ or elimination by host immune defenses.
immune modulation The deliberate attempt to change the course of an immune response, for example by altering the bias toward TH1 or TH2 dominance.
immune surveillance The recognition, and in some cases the elimination, of tumor cells by the immune system before they become clinically detectable.
immune system The tissues, cells, and molecules involved in innate immunity and adaptive immunity.
immunodeficiency diseases Any inherited or acquired disorder in which some aspect or aspects of host defense are absent or functionally defective.
immunodominant Describes epitopes in an antigen that are preferentially recognized by T cells, such that T cells specific for those epitopes come to dominate the immune response.
immunoevasins Viral proteins that prevent the appearance of peptide:MHC class I complexes on the infected cell, thus preventing the recognition of virus-infected cells by cytotoxic T cells.
immunogenic Any molecule that, on its own, is able to elicit an adaptive immune response on injection into a person or animal.
immunoglobulin (Ig) The protein family to which antibodies and B-cell receptors belong.
immunoglobulin A (IgA) See IgA.
immunoglobulin D (IgD) See IgD.
immunoglobulin domain Protein domain first described in antibody molecules but present in many proteins.
immunoglobulin E (IgE) See IgE.
immunoglobulin fold The tertiary structure of an immunoglobulin domain, comprising a sandwich of two β sheets held together by a disulfide bond.
immunoglobulin G (IgG) See IgG.
immunoglobulin-like domain (Ig-like domain) Protein domain structurally related to the immunoglobulin domain.
immunoglobulin-like proteins Proteins containing one or more immunoglobulin-like domains, which are protein domains structurally similar to those of immunoglobulins.
immunoglobulin M (IgM) See IgM.