<prompt> The immunoglobulin having highest molecular weight is:IgGIgMIgEIgA  <response>: Ans. B. Ig MImmunoglobulins, a specialized group of proteins are mostly associated with gamma globulin fraction on electrophoresis of plasma proteins. All the Ig molecules consist of two identical heavy H chains and two identical L chains. Each have constant and variable regions. Humans have five classes of Ig- IgG, IgA, IgM, IgD and IgE. Out of these IgM is the largest composed of 5-Y shaped units held together by a J polypeptide chain. Due to its largest size, IgM cannot traverse blood vessels, hence it is restricted to the blood stream. 