Gel electrophoresis of serum proteins distinguishes four major peaks (Figure 4–1). Albumin is the most abundant, normally comprising 50–70% of total serum protein. The other three are the alpha, the beta, and the gamma globulins. The gamma globulins are now called immunoglobulins (often abbreviated Ig), or antibodies (Ab). These are large, complex glycoproteins sharing a common structural motif, and grouped into classes and subclasses according to variations upon this motif. For the moment, all one need know about antibody protein structure is that a molecule of Ig contains two different polypeptides called heavy chains, and light chains. <italic>Serum protein electrophoresis</italic>. Albumin normally constitutes 65% of total serum protein. The remaining proteins are classified as α, β, or γ globulins based on electrophoretic mobility. Antibodies (immunoglobulins) are γ globulins.