Antibodies are proteins synthesized and secreted by B cells that bind to antigens. Most antigens aremacromolecules: proteins, polysaccharides, even DNA and RNA.
The 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.
http://www.arabslab.com/vb/attachmen...1&d=1234094898
These photos show one type of interaction — precipitation — between antibodies and antigen.
(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.
http://www.arabslab.com/vb/attachmen...1&d=1234094911
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.
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