- Immunoglobulins are bifunctional proteins that must interact with a small number of specialized molecules like Fc receptors and complement proteins, while also recognizing an infinite array of antigenic determinants. - They achieve this through a domain structure that provides both structural conservation and infinite variability. Immunoglobulin domains form a beta-barrel structure and are found in many proteins beyond antibodies. - Within the variable regions, hypervariable loops provide the diversity in antigen recognition, supported by a conserved framework structure. The combination of loops and framework allows recognition of countless antigens.