At its core, Fcab consists of a homodimeric Fc region with a strategically engineered antigen-binding site located on each CH3 domain. The Fc region originates from human IgG1, retaining the ability to bind to Fc receptors (FcRs) and neonatal Fc receptor (FcRn). These interactions mediate the effector functions and serum half-life of Fcab, respectively. The antigen-binding site is carefully crafted by introducing mutations into the C-terminal loops of the CH3 domain, specifically the AB-loop, the CD-loop, and the EF-loop. By modifying=these loops, different antigen-binding residues can be accommodated without compromising the integrity of the Fc structure.