Antibodies, also sometimes referred to as immunoglobulins, are large proteins (~150 kDa) with a characteristic structure often described as resembling the letter “Y”. Its stem, i.e. the lower part of the “Y”, is denoted the Fc region (crystallizable fragment) while the arms, i.e. upper part of the “Y”, are denoted as the Fab region (antigen binding fragment). The outermost part of each arm contains a so called paratope which binds the antigen, and the corresponding binding part of the antigen is called epitope. The interaction between the paratope and the epitope is highly specific, often described as a lock-and-key type of fit. See Figure 1.