G-2 glossary
antibiotic [Gk. anti, against] A substance that kills or inhibits
the growth of microorganisms.
antibody Any of a variety of Y-shaped receptor molecules
with binding sites for specific antigens. Only B cells produce
antibodies, then position them at their surface or secrete
them.
antibody-mediated immune response The B cell defensive
response to pathogens in the body wherein antibodies are
produced.
anticodon In a tRNA molecule, a sequence of three nucleo-
tide bases that can pair with an mRNA codon.
antigen (an-tih-jen) [Gk. anti, against, and genos, race,
kind] Substance that is recognized as foreign to the body and
that triggers an immune response. Most antigens are protein
molecules at the surface of infectious agents or tumor cells.
antigen–MHC complex Unit consisting of fragments of an
antigen molecule bound to MHC proteins. MHC complexes
displayed at the surface of an antigen-presenting cell such as
a macrophage promote an immune response by lymphocytes.
antigen-presenting cell A macrophage or other cell that
displays antigen–MHC complexes at its surface and so
promotes an immune response by lymphocytes.
antioxidant A chemical that can give up an electron to a free
radical before the free radical damages DNA or some other
cell constituent.
anus Terminal opening of the gastrointestinal tract.
aorta (ay-or-tah) [Gk. airein, to lift, heave] Main artery of
systemic circulation; carries oxygenated blood away from the
heart to all body regions except the lungs.
aortic body Any of several receptors in artery walls near the
heart that respond to changes in levels of carbon dioxide and
oxygen in arterial blood.
aortic valve Valve that opens from the left ventricle into the
aorta.
apoptosis (a-poh-toe-sis) Genetically programmed cell
death. Molecular signals lead to self-destruction in body cells
that have finished their prescribed functions or have become
altered, as by infection or transformation into a cancerous cell.
appendicular skeleton (ap-en-dik-yoo-lahr) Bones of the
limbs, hips, and shoulders.
appendix A slender projection from the cup-shaped pouch
(cecum) at the start of the colon. It contains lymphoid tis-
sue where defenses against pathogens in ingested food are
mobilized.
appetite The desire to eat, apart from the physical need for
food.
arteriole (ar-teer-ee-ole) Any of the blood vessels between
arteries and capillaries. They are control points where the
volume of blood delivered to different body regions can be
adjusted.
artery Any of the large-diameter blood vessels that conduct
deoxygenated blood to the lungs and oxygenated blood to all
body tissues. The thick, muscular artery wall allows arteriesaging See senescence.
aldosterone (al-doss-tuh-roan) Hormone secreted by the
adrenal cortex that helps regulate sodium reabsorption by the
kidneys.
allantois (ah-lan-twahz) [Gk. allas, sausage] One of four
extraembryonic membranes that form during embryonic
development. In humans, it functions in early blood forma-
tion and development of the urinary bladder.
allele (uh-leel) For a given location on a chromosome, one
of two or more slightly different chemical forms of a gene
that code for different versions of the same trait.
allergen Any normally harmless substance that provokes
inflammation, excessive mucus secretion, and other immune
responses.
allergy An immune response made against a normally harm-
less substance.
alveolus (al-vee-uh-lus), plural: alveoli [L. alveus, small cav-
ity] Any of the many cup-shaped, thin-walled outpouchings
of respiratory bronchioles. A site where oxygen diffuses from
air in the lungs to the blood, and carbon dioxide diffuses
from blood to the lungs.
amine hormone A hormone derived from the amino acid
tyrosine.
amino acid (uh-mee-no) A small organic molecule having
a hydrogen atom, an amino group, an acid group, and an
R group covalently bonded to a central carbon atom. The
subunit of polypeptide chains, which represent the primary
structure of proteins.
amnesia A loss of fact memory.
amniocentesis Test of fetal cells in a sample of amniotic
fluid for evidence of birth defects.
amnion (am-nee-on) One of four extraembryonic mem-
branes. It becomes a fluid-filled sac in which the embryo (and
fetus) can grow, move freely, and be protected from sudden
temperature shifts and impacts.
anabolism A metabolic activity that assembles small mol-
ecules into more complex molecules that store energy.
anal canal The canal from the rectum to the anus through
which feces pass.
analogous structures Body parts, once different in separate
lineages, that were put to comparable uses in similar environ-
ments and that came to resemble one another in form and
function. They are evidence of morphological convergence.
anaphase (an-uh-faze) The stage at which microtubules
of a spindle apparatus separate sister chromatids of each
chromosome and move them to opposite spindle poles. Dur-
ing anaphase I of meiosis, the two members of each pair of
homologous chromosomes separate. During anaphase II, sis-
ter chromatids of each chromosome separate.
anaphylactic shock A whole-body allergic response in
which a person’s blood pressure plummets, among other
symptoms.
anorexia nervosa Eating disorder in which a person deliber-
ately starves and may become dangerously thin.
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