This process allows T cells to develop self-tolerance
(distinguishing self from nonself). While developing in
the thymus gland, any T cell that reacts to the thymus’s
major histocompatibility complex (MHC) is eliminat-
ed. Tcells that tolerate the MHC learn to cooperate
with cells expressing MHC molecules and are allowed
to mature and leave the thymus.
The result is that surviving T lymphocytes tolerate
the body’s cells and cooperate with them when needed.
However, some T lymphocytes lose this ability to dif-
ferentiate self from nonself, which results in the
autoimmune diseases such as systemic lupus erythe-
matosus or multiple sclerosis.
thyroid gland Atwo-lobed endocrine gland located
at the base of the neck that secretes hormones such as
triiodothyronine (T3), thyroxine (T4), and calcitonin, a
hormone produced in the C cells that helps regulate
blood calcium by slowing down the amount of calcium
released from the bones; controls the rate of
metabolism. An enlarged thyroid gland is called goiter
and is treated with thyroid hormones.
thyroid-stimulating hormone (TSH) TSH is a hor-
mone secreted by the pituitary gland that stimulates the
synthesis and secretion of T4 (thyroxine) and T3 (tri-
iodothyronine) by the thyroid gland. TSH is itself stim-
ulated by another hormone, thyroid-releasing hormone
(TRH), which is released by the hypothalamus.
tick paralysis Ticks produce a toxin that can cause
paralysis in animals, especially if the bite is near the
spine and causes ascending paralysis. It can be reversed
if the offending tick is removed. Can be fatal if not
removed.
tight junction A cell-to-cell junction that forms a
fluid-tight seal between cells, preventing leakage or
passage of molecules from one side to the other.
Ti plasmid Shortfor tumor-inducing plasmid. It is a
plasmid of Agrobacterium tumefaciensthat is responsi-
ble for inducing tumors in infected plants. These plas-
mids are also used as vectors to introduce foreign DNA
into plant cells.tissue Agrouping or layer of cells that, with their
products and intercellular material, together perform
specialized and specific functions. Examples include
connective, epithelial, glandular, muscular, nervous,
and skeletal tissue.tobacco mosaic virus This virus was the first recog-
nized viral disease. Caused by a tobamovirus, the
tobacco mosaic virus (TMV), it attacks the leaves of
many plants, causing a mosaiclike pattern of discol-
orations in the plant.tonoplast(vascular membrane) A cytoplasmic mem-
brane that surrounds the central vacuole separating the
cell sap from the cytosol.topless tree An operational scheme for ANALOG
design.torpor Aphysiological response resulting in a drop
in body temperature almost to that of the surrounding
environment and a large reduction in the metabolic
rate, heart rate, respiration, and other functions.
Hibernation is winter torpor and estivation is summer
torpor.totipotency The ability of a single cell to develop
unlike cells that will develop into a fully differentiated
organism or part.toxicity The action of poisons (including XENOBI-
OTICs)on biochemical reactions or processes in living
organisms or ecological systems. A study of this action
is the subject matter of toxicology.toxin A poisonous material that can cause damage to
living tissues.326 thyroid gland