H-2
The mouse MHC.
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 (TH) 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.
Hypoxia
Deficiency of oxygen at tissue level including the various types that
are hypoxic hypoxia, anemic hypoxia, ischemic hypoxia, histotoxic hypoxia.