Haploid That chromosome number where each chromosome is represented once. In most higher animals it is the number present in the gametes and half of the number found in most somatic cells.

Haplotype  A set of genetic determinants located on a single chromosome.

Hapten  A small molecule which can act as an epitope but is incapable by itself of eliciting an antibody response.

HART  = Hybrid-arrest translation

Harvesting  Removal of microorganisms from a culture, usually by centrifugation.

Heartburn  Pain that seems to occur in region of heart but is due to pain receptors in esophageal wall stimulated by acid refluxed from stomach.

Heat-shock response  Increased expression of a specific group of genes (hsp genes) in response to elevated temperature or other stressful treatment accompanied by decreased transcription of other genes and decreased translation of other mRNAs. This response is very widespread among both prokaryotic and eukaryotic organisms and helps the organism survive the stress.

HeLa cell  Line of human epithelial cells, derived from a human cervical carcinoma, that grows readily in culture and is widely used in research.

Helper phage  A phage that is introduced into a host cell in conjunction with a related cloning vector, in order to provide enzymes required for replication of the cloning vector.

Helper T (T_H) cell  A functional subclass of T cells which can help to generate cytotoxic T cells and cooperate with B cells in the production of antibody responses. Helper cells recognize antigen in association with Class II MHC molecules.

Hematocrit  Percentage of total blood volume occupied by blood cells.

Hemorrhage  Bleeding.

Hemostasis  Stopping blood loss from a damaged vessel.

Heparin  Anticlotting agent found on endothelial-cell surfaces that binds antithrombin III to tissues and is used as an anticoagulant drug.

Heterochromatin  Regions of chromatin that remain highly condensed and transcriptionally inactive during interphase.

Heterokaryon Genetically different nuclei in a common cytoplasm, usually derived by cell fusion.

Heterologous probing  The use of a labeled nucleic acid molecule to identify related molecules by hybridization probing.

Heteroploid  A term used to describe a culture (not a cell) where the cells comprising the culture have chromosome numbers other than diploid.

Heterozygote  Referring to a diploid cell or organism having two different alleles of a particular gene.

High-density lipoprotein (HDL)  Lipid-protein aggregate having low proportion of lipid that promotes removal of cholesterol from cells.

Hinge  (on immunoglobulin molecule) The portion of an immunoglobulin heavy chain between the Fc and Fab regions which permits flexibility within the molecule and allows the two combining sites to operate independently. The hinge region is usually encoded by a separate exon.

Histamine  A major vasoactive amine and inflammatory chemical messenger released from mast cell and basophil granules. It is a monoamine neurotransmitter.

Histocompatibility  The ability to accept grafts between individuals.

Histone  A family of small, highly conserved basic proteins, found in the chromatin of all eukaryotic cells, that associate with DNA in the nucleosome. The five major types are H1, H2A, H2B, H3, and H4 histone. Histone-like proteins also are present in some prokaryotes.

Histotypic A culture resembling tissue-like morphology in vivo. It is usually implied that this is a three-dimensional culture recreated from dispersed cell culture that attempts to retain, by cell proliferation and multilayering or by reaggregation, the tissue-like structure. Organ cultures cannot be propagated, whereas histotypic cultures can.

HLA  = Human leucocyte antigen system.

Homeobox  Conserved protein sequence which forms a DNA-binding domain (homeodomain) in a class of transcription factors encoded by certain homeotic genes.

Homeo domain  A conserved DNA-binding motif found in many developmentally important transcription factors.

Homeostasis  Relatively stable condition of extracellular fluid that results from regulatory system actions.

Homeothermic Able to maintain a constant body temperature in spite of environmental fluctuation.

Homograft (Allograft)  A graft derived from a genetically different donor of the same species as the recipient.

Homokaryon  Genetically identical nuclei in a common cytoplasm, usually a product of cell fusion.

Homology  Refers to two genes, from different organisms and therefore of different sequence, that code for the same gene product. Two homologous genes ate usually sufficiently similar in sequence for one to be used as a hybridization probe for the other.

Homopolymer tailing  The attachment of a sequence of identical nucleotides (e.g. AAAAA) to the end of a nucleic acid molecule, usually referring to the synthesis of single-stranded homopolymer extensions on the ends of a double-stranded DNA molecule.

Homozygote  Referring to a diploid cell or organism having two identical alleles of a particular gene.

Horseradish peroxidase  An enzyme that can be complexed to DNA and which is used in a non-radioactive procedure for DNA labeling.

Host-controlled restriction  A mechanism by which some bacteria phage attack through the synthesis of a restriction endonuclease that cleaves coded by a cloned gene.

HRT  = Hybrid-release translation

Human leucocyte antigen (HLA) system  The human MHC.

Humoral  Pertaining to the extracellular fluids, including the serum and lymph.

Humoral immunity  Immunity conferred by circulating antibodies produced by B lymphocytes and plasma cells.

Hybrid-arrest translation (HART)  A method used to identify the polypeptide coded by a cloned gene.

Hybrid cell  Mononuclear cell that results from the fusion of two different cells, leading to the formation of a synkaryon.

Hybridization  Association of two complementary nucleic acid strands to form double-stranded molecules. Hybrids can contain two DNA strands, two RNA strands, or one DNA and one RNA strand. In situ hybridization is a technique for determining the location of a specific RNA sequence within a tissue or cell by treatment with a labeled (e.g., radiolabeled) single-stranded nucleic acid probe followed by detection (e.g., autoradiography). In situ hybridization is also used to map the location of genes to specific chromosomal locations.

Hybridoma  = Hybrid cell.

Hybrid-release translation (HRT)  A method used to identify the polypeptide coded by a cloned gene.

Hydrogen bond  Weak chemical bond between two molecules or parts of the same molecule, in which negative region of one polarized substance is electrostatically attracted to a positively polarized hydrogen atom in the other.

Hydrophilic  Interacting effectively with water.

Hydrophobic  Not interacting effectively with water; in general, poorly soluble or insoluble in water.

Hydrophobic interaction  The force that drives association of nonpolar molecules or parts of molecules with each other in aqueous solution. A type of noncovalent bond that is particularly important in stabilization of the phopholipid bilayer.

Hyper-  Increased. Too much.

Hypermia  = Active hypermia.

Hypertension  Chronically increased arterial blood pressure.

Hypertonic  A hypertonic solution is one having an osmotic strength greater than that of a cell (~300 mOsm/kg H₂O). Such a solution causes water to move out of a cell due to osmosis.

Hypertrophy  Enlargement of a tissue or organ due to increased cell size rather than increased cell number.

Hypervariable region  The most variable areas of the V domains of immunoglobulin and T-cell receptor chains. These regions are clustered at the distal portion of the V domain and contribute to the antigen-binding site.

Hypo-  Decreased. Too little.

Hypotension  Low blood pressure.

Hypotonic  A hypotonic solution has an osmotic strength lower than that of a cell (~300 mOsm). Such a solution causes water to move into a cell due to osmosis.