Antibodies are proteins synthesized and secreted by B cells that bind to
antigens. Most antigens aremacromolecules: proteins, polysaccharides, even
DNA and RNA.
interaction occurs: by noncovalent forces (like that between enzymes and
their substrate) between the antigen-combining site on the antibody and a
portion of the antigen called the antigenic determinant or epitope.
photos show one type of interaction — precipitation — between antibodies
(a) The tube contains antibodies to the Type III pneumococcal
polysaccharide isolated from the capsule surrounding the bacteria.
(b) A solution of the polysaccharide is added, and
(c) the formation of insoluble antigen-antibody complexes is revealed by
the almost instantaneous appearance of turbidity.
(d) After an hour, the complexes settle out as a precipitate. If the
proportion of antigen to antibody in the mixture is selected properly, the
fluid above the precipitate will be devoid of both.
In the human body, this binding can literally be life-saving.
The capsule that surrounds pneumococci protects them from phagocytosis. (Pneumococci
that fail to make a capsule — "R" forms — do not cause disease.
If the appropriate antibodies are present in the body, they combine with
the capsule. Coated with protein instead of polysaccharide, the
pneumococci are now easy to ingest.
These photomicrographs show phagocytosis of antibody-coated pneumococci.
Left: A neutrophil extends a pseudopod toward two pneumococci.
Center: these bacteria have been engulfed (arrows), and the neutrophil is
beginning to engulf four more pneumococci at the upper right.
Right: Two pneumococci have escaped.
(From W. B. Wood, M.
R. Smith, and B. Watson, Journal of Experimental Medicine 84:387, 1946.)
In the days before antibiotics, the start of antibody production by the
immune system of the patient marked the turning point in the progression
of the disease.