highly active antiretroviral therapy (HAART) A combination of drugs that is used to control HIV infection. It comprises nucleoside analogs that prevent reverse transcription, and drugs that inhibit the viral protease.
hinge region The ﬂexible domain that joins the Fab arms to the Fc piece in an immunoglobulin. The ﬂexibility of the hinge region in IgG and IgA molecules allows the Fab arms to adopt a wide range of angles, permitting binding to epitopes spaced variable distances apart.
HIP/PAP An antimicrobial C-type lectin secreted by intestinal cells in humans. Also known as RegIIIα.
histamine A vasoactive amine stored in mast-cell granules. Histamine released by antigen binding to IgE antibodies bound to mast cells causes the dilation of local blood vessels and the contraction of smooth muscle, producing some of the symptoms of IgE-mediated allergic reactions. Antihistamines are drugs that counter histamine action.
histatins Antimicrobial peptides constitutively produced by the parotid, sublingual, and submandibular glands in the oral cavity. Active against pathogenic fungi such as Cryptococcus neoformans and Candida albicans.
HIV See human immunodeficiency virus.
HLA The genetic designation for the human MHC. Individual loci are designated by upper-case letters, as in HLA-A, and alleles are designated by numbers, as in HLA-A*0201.
HLA-DM An invariant MHC protein resembling MHC class II in humans that is involved in loading peptides onto MHC class II molecules. A homologous protein in mice is called H-2M, or sometimes H2-DM.
HLA-DO An invariant MHC class II molecule that binds HLA-DM, inhibiting the release of CLIP from MHC class II molecules in intracellular vesicles. A homologous protein in mice is called H-2O or H2-DO.
homeostatic chemokines Chemokines that are produced at steady-state to direct the localization of immune cells to lymphoid tissues.
homing The direction of a lymphocyte into a particular tissue.
homing receptors   Receptors on lymphocytes for chemokines, cytokines, and adhesion molecules specific to particular tissues, and which enable the lymphocyte to enter that tissue.
homozygous Describes individuals that have two identical alleles of a given gene, inherited separately from each parent.
host-versus-graft disease (HVGD) Another name for the allograft rejection reaction. The term is used mainly in relation to bone marrow transplantation when immune cells of the host recognize and destroy transplanted bone marrow or hematopoietic stem cells (HSCs).
human immunodeficiency virus (HIV) The causative agent of the acquired immune deficiency syndrome (AIDS). HIV is a retrovirus of the lentivirus family that selectively infects macrophages and CD4 T cells, leading to their slow depletion, which eventually results in immunodeficiency. There are two major strains of the virus, HIV-1 and HIV-2, of which HIV-1 causes most disease worldwide. HIV-2 is endemic to West Africa but is spreading.
human leukocyte antigen (HLA) See HLA.
humanization The genetic engineering of mouse hypervariable loops of a desired specificity into otherwise human antibodies for use as therapeutic agents. Such antibodies are less likely to cause an immune response in people treated with them than are wholly mouse antibodies.
humoral    Referring to effector proteins in the blood or body ﬂuids, such as antibodies in adaptive immunity, or complement proteins in innate immunity.