The forks of the Y react with the different antigens. The base of the Y determines the vehicle from which the antibodies function - the classes of antibodies. There are several such classes of antibodies, and the cells producing the antibodies have considerable flexibility to change the class over time. The V-region in humans codes for 95 amino acids of the final polypeptide. It is coded into 150 exons each coding for 95 amino acids, but have a diversity of particular amino acids in the composition.
Different species have different numbers of V-regions.
This is analogous of the processing of hnRNA, but involves the double stranded DNA strand. Once modified in the DNA, future cells produced by mitosis form a clone of cells that can only produce the kinds of antibodies limited by this genetic specialization. In order to accomplish this genetic excision of most of the antibody locus, and the associated potential for diversity in antibody production, the single strand individually folds on itself , and a long stretch is removed very precisely. A mistake of only one nucleotide would cause a frame shift mutation, and inactivate the locus. Indeed, this happens, and, unless repaired, the affected locus has no possible antibody formation function.