milk, and mucous, both intestinal and bronchial. They
are secreted on the surface of the body as a first defense
against bacteria and viral antigens in an attempt to pre-
vent them from entering the body.
IgM, or immunoglobulin M, antibodies are pro-
duced in response to new or repeat infections and stay
in the body for a short time after infection. They make
up from 5 to 10 percent of the total immunoglobulins
and are the first to show up in the serum after an anti-
gen enters the system. IgM is produced during the pri-
mary immune response. It is the IgMs that capture and
bind antigens to form large insoluble complexes that
are cleared from the blood.
IgG, or immunoglobulin G (gamma globulin), anti-
bodies remain in the body for long periods of time after
infection and are the most common type, comprising
about 80 percent of the body’s total immunoglobulins.
They are in the serum and are produced in substantial
quantities during the secondary immune response, and
along with IgM, they activate the complement system,
which results in the destruction of pathogen membranes.
The IgGs act by agglutinating, by opsonising, and by
activating complement-mediated reactions against cellu-
lar pathogens and by neutralizing toxins.
IgE, or immunoglobulin E, is associated with mast
cells, which are basophils, a type of granular white
blood cell that has left the bloodstream and entered a
tissue. Mast cells release histamine and heparin, chemi-
cals that mediate allergic reactions. Not surprisingly,
IgE is responsible for immediate hypersensitivity (aller-
gic) reactions and immune defense against parasites.
IgD, or immunoglobulin D, is a specialized immuno-
globulin, but its function is currently unknown. It is
found in small amounts in the serum.
antibonding orbital A molecular orbital higher in
energy than any of the atomic orbitals from which it is
derived; creates instability to a molecule or ion when
populated with electrons.
antiferromagnetic SeeFERROMAGNETIC.
antigen A foreign substance, a macromolecule, that
is not indigenous to the host organism and therefore
elicits an immune response.
anti-Markownikoff addition antigen A foreign
substance, a macromolecule, that is not indigenous to
the host organism and therefore elicits an immune
response.
See alsoMARKOWNIKOFF RULE.antimetabolite A structural ANALOGof an interme-
diate (substrate or COENZYME) in a physiologically
occurring metabolic pathway that acts by replacing the
natural substrate, thus blocking or diverting the
biosynthesis of physiologically important substances.antisense molecule An OLIGONUCLEOTIDEor ANALOG
thereof that is complementary to a segment of RNA
(ribonucleic acid) or DNA (deoxyribonucleic acid) and
that binds to it and inhibits its normal function.aphotic zone The deeper part of the ocean beneath
the photic zone, where light does not penetrate suffi-
ciently for photosynthesis to occur.
See alsoOCEANIC ZONE.apoprotein A protein without its characteristic PROS-
THETIC GROUPor metal.aprotic solvent NonPROTOGENIC(in a given situa-
tion). With extremely strong BRONSTED ACIDs or
BRONSTED BASEs, solvents that are normally aprotic
may accept or lose a proton. For example, acetonitrile
is in most instances an aprotic solvent, but it is PRO-
TOPHILICin the presence of concentrated sulfuric acid
and protogenic in the presence of potassium tert-
butoxide. Similar considerations apply to benzene,
trichloromethane, etc.
See alsoDIPOLAR APROTIC SOLVENT.aquation The incorporation of one or more integral
molecules of water into another chemical species with
or without displacement of one or more other atoms or
groups.
See alsoHYDRATION.16 antibonding orbital