The answers are probably not nearly as complicated as some would make
it seem.
The genes encoding antibodies are found as several distinct loci on
chromosomes. During the ontogeny of B cells, the cell randomly
selects one DNA sequence from each loci, and ligates them together.
The rest of the sequences which were not used are either spliced out
or are no longer functional. After the gene sequences have been
selected, a functional antibody is produced.
The catch is that this process is irreversible, and the cell has only
one chance to make a functional antibody due to the cutting up of the
loci which contribute the sequences.
The resulting cell expresses a single antibody for the rest of it's
life span. It can and often does change the class of antibody
produced, but the specificity is fixed.
The body gets around the problem by producing millions of new B cells
daily, each with a uinique, randomly selected specificity. This
process has served us well in evolutionary terms.
Jay Mone'
