Publisher Summary This chapter presents the results obtained in the investigation of the three-dimensional structure of a human Fab´ fragment currently. The chapter explores preliminary results of X-ray diffraction studies of immunoglobulins. X-ray diffraction analysis of single crystals can provide a high-resolution model of a molecule by a Fourier series representation of the electron density, calculated from experimental diffraction data and plotted as a contour map. The four homology subunits of Fab´ closely resemble each other, sharing a basic pattern of polypeptide chain folding or immunoglobulin fold. The arrangement of the Fab´ subunits, linked by narrow stretches of polypeptide chain, suggest flexibility and a potential for conformational changes by a relative movement of the homology subunits. The existence of this underlying structural pattern further supports the postulate that a gene-duplication mechanism gave rise to the subunit structure of immunoglobulins. The chapter concludes that the knowledge of the structure of immunoglobulins per se is of great interest to immunologists and immunochemists and such knowledge will provide a frame of reference for detailed correlations between structure and function and for investigations of physiological reactions and mechanisms of the immune response.