Basic Structure of Antibodies

Blood can be separated in a centrifuge into a fluid and a cellular fraction. The fluid fraction is the plasma and the cellular fraction contains red blood cells, leukocytes, and platelets. Plasma contains all of the soluble small molecules and macromolecules of blood, including fibrin and other proteins required for the formation of blood clots. If the blood or plasma is allowed to clot, the fluid phase that remains is called serum. It has been known since the turn of the century that antibodies reside in the serum. The first evidence that antibodies were contained in particular serum protein fractions came from a classic experiment by A. Tiselius and E. A.Kabat, in 1939. They immunized rabbits with the protein ovalbumin (the albumin of egg whites) and then divided the immunized rabbits’ serum into two aliquots. Electrophoresis of one serum aliquot revealed four peaks corresponding to albumin and the alpha (), beta (), and gamma () globulins. The other serum aliquot was reacted with ovalbumin, and the precipitate that formed was removed; the remaining serum proteins, which did not react with the antigen, were then electrophoresed. A comparison of the electrophoretic profiles of these two serum aliquots revealed that there was a significant drop in the -globulin

peak in the aliquot that had been reacted with antigen (Figure 4-1). Thus, the -globulin fraction was identified as containing serum antibodies, which were called immunoglobulins, to distinguish them from any other proteins that might be contained in the -globulin fraction. The early experiments of Kabat and Tiselius resolved serum proteins into three major nonalbumin peaks—,  and .We now know that although immunoglobulin G (IgG), the main class of antibody molecules, is indeed mostly found in the -globulin fraction, significant amounts of it and other important classes of antibody molecules are found in the  and the  fractions of serum.