Antigens
Natural
immunity
depends
upon
the
presence
of
natural
antibodies
(modified
blood
globulins)
which
can
react
with
certain
substances
called
antigens.
Antigens
are
substances
which
are
capable
of
stimulating
immune
responses.
They
are
foreign
macromolecules
which
can
stimulate
antibody
formation.
The
ability
of
a
substance
to
stimulate
antibody
formation
is
called
its
antigenicity.
Antigens
include
most
proteins,
Some
polysaccharides
(e.
g.
in
bacterial
cell
walls),
nucleic
acid
and
complex
lipids
and
virus
particles.
They
are
high
molecular
weight
substances,
with
a
molecular
weight
generally
greater
than
5,000.
