Fig. (2) The variable (V) domains of heavy (H) and light (L) chains of antibodies (Abs) are assembled during B cell development in the bone marrow. The VH chains are composed of a V, a Diversity (D), and a Joining (J) region that are encoded in separate, non-contiguous gene segments. The VL is defined by a different set of Vs and Js. The V and C domains of the H and L chains combine in a quaternary structure that places the antigen-binding (Fab, paratope) sequences or complementarity-determining regions (CDR1-3) of the H and L chains at the same end of the Ab. The VH CDR3 domain is the most variable of the CDRs providing most of the Abs specificity [6]. Non-antigen selected, naïve B cells can express CDRs that may have three contact residues (white dots) and a micromolar binding constant (K_d). Affinity matured Abs have somatic hypermutated rearranged Ig genes providing additional antigen contact points (white dots) resulting in increased affinity (nanomolar K_d) Ab. An scFv (single chain, fraction variable) phage product represents an Fab-like molecule (VH/VL) without the proximal C domains.