insulin A protein hormone produced in the pan-
creas by beta cells, located in the islets of Langerhans,
that stimulates cellular utilization of glucose by body
cells, by converting glucose and other carbohydrates
toenergy, and helps control blood-sugar levels by act-
ing antagonistically with glucagons, the chief source
ofstored fuel, in the liver. It is released by various sig-
nals that are sensitive to the intake and digestion of
food. It also acts as an important regulator of protein
and lipid metabolism. Insulin is used as a drug to con-
trol insulin-dependent diabetes mellitus, a disorder
that is caused by the insufficient production of
insulin. Without insulin, cells do not absorb glucose.
Diabetic individuals can have type I diabetes (juvenile)
comprising about 10 percent of the population, or
type II diabetes (adult). Presently some 16 million
Americans have diabetes, with 1,700 new cases being
diagnosed daily. Diabetes has been linked to the devel-
opment of a variety of diseases including heart dis-
ease, stroke, peripheral vascular disease, and
neurological disorders.
integral protein Aprotein of biological membranes
that penetrates into or spans the lipid bilayer of a cell
membrane. The lipid bilayer is a protective membrane
that surrounds the cell and consists of two layers of
phospholipids.
integrated pest management(IPM) SeeBIOLOGICAL
CONTROL.
inter- A prefix meaning between or among.
interbreed (crossbreed) To breed with another kind
or species; hybridize.
intercalation compounds Compounds resulting
from inclusion, usually without covalent bonding, of
one kind of molecule (the guest molecule) in a matrix
of another compound (the host compound), which has
a layered structure. The host compound, with a rather
rigid structure, may be macromolecular, crystalline, or
amorphous.
interferon A chemical messenger of the immune sys-
tem, composed of a group of cytokine proteins that
have antiviral characteristics that are capable of help-
ing the immune response. Three main types of interfer-
on—alpha, beta, and gamma—are produced by
virus-infected cells and are released to coat uninfected
cells, thus preventing them from becoming infected.
Alpha interferon is produced by virus-infected mono-
cytes and lymphocytes, while beta is produced by virus-
infected fibroblasts. Gamma is produced by stimulated
T and NK cells.interleukin Agroup of natural chemical glycopro-
tein messengers, acting as cytokines, that are secreted
by different cells of the immune system to make other
cells perform specific cellular functions. Interleukin-1
(IL-1) is released early by monocytes, macrophages, T
cells, and other immune cells to fight infection. It
stimulates T cell proliferation and protein synthesis
and causes fever. Can be cleaved into a peptide
involved in cell death (apoptosis). Interleukin-2 (IL-2)
is produced by T helper and suppresser lymphocytes.
IL-2 increases the expression of natural killer and
other cytotoxic cells and stimulates helper T cells to
proliferate more rapidly. IL-2 is produced commercial-
ly by recombinant DNA technology and used for the
treatment of metastatic renal (i.e., kidney) cell cancer.
Interleukin-4 (IL-4) is released by lymphocytes (TH-2
subset of T helper lymphocytes) and promotes anti-
body production by stimulating B cells to proliferate
and mature and promotes allergic responses via pro-
duction of the immunoglobulin IgE. Interleukin-6 (IL-
6) affects many different cells in the immune system
by inducing differentiation and activation. Inter-
leukin-10 (IL-10), like IL-4, is released by lympho-
cytes (the TH-2 subset of T helper lymphocytes).
IL-10 enhances the humoral response and increases
antibody production. Interleukin-12 (IL-12) induces
the production of natural killer and other cytotoxic
immune cells.intermediate filament A fibrous protein filament of
the cytoskeleton that helps form ropelike bundles in
animal cells and is about 10 nm in size, falling in the
middle between the size of microtubules and microfila-
ments; provides tensile strength.178 insulin