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vaccination Injection of a killed or weakened infec-
tious organism (virus, bacterium) in order to prevent
the disease. Vaccinations are administered via needles,
orally, or by aerosol spray.

vaccine A preparation that stimulates an immune
response in the body that can prevent an infection or
create resistance to an infection or disease. Vaccines are
administered via needles, orally, or by aerosol spray.
There are several types of vaccines, ranging from
monovalent types that contain only one antigen to
combinational vaccines where several antigens are com-
bined into one. Vaccines can be live or attenuated, i.e.,
a weakened strain, to induce the immune response
while bypassing the severe effects of the disease. Com-
mon vaccines include measles, mumps, polio, and oth-
ers. A live-vector vaccine uses a nondisease virus or
bacterium to deliver a foreign substance to develop
immunity. Aninactive vaccine contains dead viruses or
bacteria and cannot cause disease but will trigger a
response.
There are acellular vaccines that contain only a
partial amount of cellular material; DNA vaccines
that inject genes with coding for a specific antigen
protein; bacterial vaccines that use bacteria; intracel-
lular vaccines within a cell; and conjugate vaccines
that are made with polysaccharide (carbohydrate)
antigens bound to proteins to improve the effect of
immunity.

vacuole Alarge membrane-bound, fluid-filled space

within a cell. In plant cells, there usually is a single

large vacuole filling most of the cell’s volume, which

helps maintain the shape of the cells. Vacuoles can con-

tain food, gas, ingested bacteria, and other debris. In

species such as the paramecium, two vacuoles are

important: the contractile vacuole is used in osmoregu-

lation, which is the removal of excess water; the food

vacuole contains recently ingested food, where it even-

tually combines or fuses with the cell’s lysosomes,

which contain enzymes for digestion.

vagility Free to move about, ability to move or

migrate. A vagile species is one whose distribution can

vary widely from year to year.

valence shell Valence electrons are the electrons

located in the outermost, highest energy orbits or

“shell” of an atom. The “shell” is more of a field

density and indicates the region where the electrons are

located. The valence electrons determine the chemical

properties of an element, since it is these valence elec-

trons that are gained or lost during a chemical reaction.

van der Waals forces Weak forces, such as seen in

hydrogen bonding, that contribute to intermolecular

bonding.

V