118 Self and the Phenomenon of Life
b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanity “9x6”key” manner dictates how strong and specific the binding is. The stron-
ger the binding, the more effective is the antibody. Antibody-antigen
binding takes place within a stretch of ten amino acids in a protein mol-
ecule; sometimes an antibody can tell the difference between two anti-
gens that differ in only one amino acid. Figure 6.2 shows schematically
the structure of an antibody.
Common sense leads to the thinking that antibodies derive their
specificity following contact with the antigens, but this is not quite the
case. The surprising fact is that a repertoire of antibodies, covering an
enormously wide range of specificity, is ready-made by the naïve B cells
before they are even exposed to any foreign antigen. This is achieved by
a unique way of random gene recombination occurring in the nucleus
of the lymphocytes. By one estimate, an animal can generate a poten-
tial repertoire of 10^12 specific antibodies, enough to bind more types of
antigens than it can ever expect to encounter in a lifetime. On the other
hand, each lymphocyte is uni-potent, i.e., it is dedicated to producing
Fig. 6.2. Schematic view of a typical antibody, IgG (immunoglobulin G). Antibodies
are proteins made up of amino acid chains. IgG comprises a pair of heavy chains and a
pair of light chains. Both chains contribute to the antigen-binding sites.