Immunology | Immune system| Immunity

            Blood can be separated in a centrifuge into a fluid and a cellular fraction contains red blood cells,leukocytes,and platelets.Plasma contains all 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 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 white)and then divided the immunized rabbits serum intoo two aliquots.Electrophoresis of one serum aliquot revealed four peaks corresponding to albumin and 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 electrophoressed.A comparison of the electrophoretic profiles of these two serum aliquots revealed that there was a significant drop in the gamma-globbulin peak in the aliquot that had been reacted with antigen.Thus,the gamma-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 gamma-globulin fraction.We now know that although immunoglobulin G(IgG),the main class of antibody molecules are found in the gamma-globulin fraction,significant amounts of it and other important classes of antibody molecules are found in the alpha and the beta fractions of serum.