Antibodies are serum proteins which are called immunoglobulins (IgA, IgG, IgM, IgD, IgE). All antibodies have the same basic structure. Every antibody molecule has two identical light chains and two identical heavy chains. Antibody isotypes are determined by their heavy chains: the five types of antibody classes are defined by five different heavy chains. The regions of the antigen binding site are formed by the variable light chains (VL) and the variable heavy chains (VH). Genetic mechanisms create antibody diversity at the somatic level in a step-wise process by recombination of a set of gene segments (VL and VH domains) and hypermutation events during which the B cell maturation occurs. In the course of differentiation and maturation processes, antibody genes are transcribed into messenger RNA, and, specifically, antibodies are tailored by multiple gene splicings and gene rearrangements.