There are five classes of immunoglobulins in Human and mouse: IgA, IgD, IgE, IgM, and IgG. So far, four istotypes have been identified (IgA, IgE, IgG, and IgM) in rabbit. IgG is the most abundant immunoglobulin and is equally distributed in blood and tissue. Rabbit has only one IgG subclass,The general immunoglobulin structure is composed of four polypeptide chains, two heavy and two light chains linked together and to each other by disulfide bonds, creating a tetrameric quaternary structure. The resulting tetramer creates two identical halves which together form a Y like structure. While the amino-terminal portions that exhibits highly variable amino-acid composition are involved in antigen binding, the C terminal constant parts are involved in complement binding, placental passage and binding to cell membrane. IgG is involved in response to a foreign antigen. The presence of IgG usually signifies a mature antibody response. IgG has a molecular weight of about 150 kDa, it can bind to many pathogens and also plays an important role in antibodydependent cell-mediated cytotoxicity. Typically rabbit serum and plasma samples contain about 5 to 10 mg/ml of IgG. The rabbit immune system has been documented as a vehicle for developing antibodies with higher affinity and more diverse recognition of many molecules including phospho-peptides, carbohydrates and immunogens that are not otherwise immunogenic in mouse. The main type of antibodies available from rabbit is polyclonal antibodies. In recent years, the use of hybridoma technology for the production of rabbit monoclonal antibodies has created the need for a fast and simple procedure for quantifying antibody production in vitro (e.g., culture supernatant) or in vivo (e.g., ascites). An accurate determination of immunoglobulin levels in the serum, plasma or hybridoma-culture supernatant is essential to study the effect of drugs or physical parameters on the level of immunoglobulin secretion or hybridoma growth.