humoral immunity, humoral immune response Immunity due to proteins circulating in the blood, such as antibodies (in adaptive immunity) or complement (in innate immunity). Adaptive humoral immunity can be transferred to unimmunized recipients by the transfer of serum containing specific antibody.
HVGD See host-versus-graft disease.
hydrophobic interaction Chemical interaction occurring between nearby hydrophobic moieties typically excluding water molecules.
21-hydroxylase An enzyme of non-immune function but encoded in the MHC locus required for normal cortisol synthesis by the adrenal gland.
3-hydroxy-3-methylglutaryl-co-enzyme A (HMG-CoA) reductase Rate-limiting enzyme in the production of cholesterol and a target of cholesterol-lowering drugs such as the statins.
hygiene hypothesis A hypothesis first proposed in 1989 that reduced exposure to ubiquitous environmental microorganisms was a cause of the increased frequency of patients with allergies observed over the course of the midto late-20th century.
hyper IgE syndrome (HIES) Also called Job’s syndrome. A disease characterized by recurrent skin and pulmonary infections and high serum concentrations of IgE.
hyper IgM syndrome A group of genetic diseases in which there is overproduction of IgM antibody, among other symptoms. They are due to defects in various genes for proteins involved in class switching such as CD40 ligand and the enzyme AID. See activation-induced cytidine deaminase, CD40 ligand deficiency.
hyper IgM type 2 immunodeficiency See activation-induced cytidine deaminase.
hyperacute graft rejection Immediate rejection reaction caused by preformed natural antibodies that react against antigens on the transplanted organ. The antibodies bind to endothelium and trigger the blood-clotting cascade, leading to an engorged, ischemic graft and rapid death of the organ.
hypereosinophilic syndrome Disease associated with an overproduction of eosinophils.
hypervariable regions See complementarity-determining regions.
hypomorphic mutations Applied to mutations that result in reduced gene function.
IκB A cytoplasmic protein that constitutively associates with the NFκB homodimer, composed of p50 and p65 subunits. When IκB is phosphorylated by activated IKK (IκB kinase), IκB becomes degraded and allows the NFκB dimer to be released as an active transcription factor.
IκB kinase (IKK) See IKK.
iC3b Inactive complement fragment produced by cleavage of C3b.
ICAMs   ICAM-1, ICAM-2, ICAM-3. Cell-adhesion molecules of the immunoglobulin superfamily that bind to the leukocyte integrin CD11a:CD18 (LFA-1). They are crucial in the binding of lymphocytes and other leukocytes to antigen-presenting cells and endothelial cells.
ICOS (inducible co-stimulatory) A CD28-related co-stimulatory receptor that is induced on activated T cells and can enhance T-cell responses. It binds a co-stimulatory ligand known as ICOSL (ICOS ligand), which is distinct from the B7 molecules.
ICOSL See ICOS.
IFI16 (IFN-γ-inducible protein 16) A member of the PYHIN subfamily of NLR (NOD-like receptor) family containing an N-terminal HIN domain. It activates the STING pathway in response to double-stranded DNA.
IFIT (IFN-induced protein with tetratricoid repeats) A small family of host proteins induced by interferons that regulate protein translation during infection in part by interactions with eIF3.