taxonomy The name of the discipline given to the
scientific study, naming, classification, and identifica-
tion of organisms.
T cell(T-lymphocyte) A lymphocyte (white blood
cell) that develops in the bone marrow, matures in the
thymus, and expresses what appear to be antibody
molecules on its surface. Unlike B cells, these molecules
cannot be secreted. Also called a T-cell receptor (CD3
and CD4 or CD8). Works as part of the immune sys-
tem in the body. Produces cytokine to help B lympho-
cytes produce immunoglobulin.
Several distinct T-cell subpopulations are recognized:
helper T cell (T4 cell, CD4 cell)A class of T cells that
help trigger B cells to make antibodies against thy-
mus-dependent antigens. Helper T cells also help
generate cytotoxic T cells.
Tkiller cells (cytotoxic T cells)Cells that kill target
cells bearing appropriate antigen within the groove
of an MHC (major histocompatibility complex)
class I molecule that is identical to that of the T cell.
suppressor T cell (T8 cell, CD8 cell)The existence of
these cells is a relatively recent discovery, and their
function is still somewhat debated. The basic con-
cept of suppressor T cells is a cell type that specifi-
cally suppresses the action of other cells in the
immune system, notably B cells and T cells, thereby
preventing the establishment of an immune
response. How this is done is not known with cer-
tainty, but it seems that certain specific antigens can
stimulate the activation of the suppressor T cells.
This suppressor effect is thought to be mediated by
some inhibitory factor secreted by suppressor T
cells. The factor is not any of the known lym-
phokines. Another fact that renders the study of this
cell type difficult is the lack of a specific surface
marker. Most suppressor T cells are CD8-positive, as
are cytotoxic T cells.
telomerase (telomere terminal transferase) An
enzyme,known as an immortalizing enzyme, that is
composed of RNA and proteins that uses its RNA as a
template to synthesize telomeric DNA onto the ends of
chromosomes; acts as a reverse transcriptase, adding
telomeres to the chromosome ends when activated.
Found in germ and inflammatory cells, fetal tissue, and
tumor cells. When cells become cancerous, telomerase is
activated, and the cells can replicate without limits in a
process called immortalization.telomere The protective and stabilizing ends of lin-
ear chromosomes in eukaryotes that are involved in the
replication and stability of DNA molecules. Telomeres
erode slightly with each cell division unless TELO-
MERASEis activated.telophase The last stage in meiosis. Telophase I is the
stage where the migration of the daughter chromo-
somes to the two poles is completed. In telophase II,
the last stage of meiosis, a nuclear membrane forms
around each set of chromosomes, and cytokinesis (divi-
sion of the cytoplasm) takes place.temperate deciduous forest Located in the midlati-
tude areas between the polar regions and the tropics.
This biome has four seasons, the result of being
exposed to warmand cold air masses. The temperature
varies widely from season to season, with cold winters
and hot, wet summers. Most of the trees are broadleaf
trees such as oak, maple, beech, hickory, and chestnut.
Animals have adapted to the seasons through a variety
of strategies, including hibernating, migrating, or stay-
ing active.temperate virus A virus that does not necessarily kill
the host by lysis but that reproduces in synchrony with
the host, called a lysogen. When this process is occur-
ring the virus is called a prophage.temperature A measure of the energy in a substance.
The more heat energy in the substance, the higher the
temperature. A number of temperature scales have
evolved over time, but only three are used presently:
Fahrenheit, Celsius, and Kelvin. The Fahrenheit tem-
perature scale is a scale based on 32 for the freezing
point of water and 212 for the boiling point of water,
the interval between the two being divided into 180
parts. This scale is named after its inventor, the 18th-322 taxonomy