This amazing variability is due the fact that immunoglobulin domains have built-in genetic variability. During the maturation of antibody-producing cells, the multiple copies of variable segments V, joining segments J and diversity segments D are spliced together on DNA level to produce in each clone of the B-cell a unique combination. Inprecisions in the splicing process produce ever greater variability, and a high rate of somatic mutations allow the affinity maturation of antibodies. The resulting variable domains of the heavy chains are then joined to different constant domains to produce the differnt antibody subtypes:Mü or Delta constant chains to produce Cell-surface bound B-cell receptors,Alpha and Gamma chains for secreted antibodies.

This unique mechanism allows the immune system to evolve antibodies to the most diverse antigens within a very short timespan
The regions of variability predisposed for antigen contact are called "Complementartity Determining Regions" or CDRs)

V_L-Domain: yellow: CDR L1, orange: CDR L2, red: CDR L3
V_H-Domain: green: CDR H1, cyan: CDR H2, blue: CDR H3