Most antibodies are Y shaped and comprises four subunits, two heavy chains and two light chains. Each light chain and a moiety of each heavy chain pairs together via one disulfide bond to form the upper branch of ‘Y’, also named as Fab(antigen-binding fragment). The rest part of two heavy chain form the root of ‘Y’ via two disulfide bonds, also named Fc. Collectively, one antibody has two Fabs and one Fc. The top part (N termini) of Fab are the variable domains of light chain (VL) and heavy chain (VH) with the antigen-binding site on the tip, and the rest part, involving some part of Fab and the whole part of Fc, is the more constant domain (CL and CH). Accordingly, Fab decides the specificity and affinity of antibody to antigen while Fc determines the locations of antibody.