Harpers

PLASMA PROTEINS & IMMUNOGLOBULINS / 595HLMonomerDimerLHA. Serum lgAHLJ chainHLLHB. Secretory lgA(dimer)J chainSecretorycomponentHLFigure 50–10. Schematic representationof serum IgA, secretoryIgA, and IgM. Both IgA and IgM havea J chain, but only secretory IgA hasa secretory component. Polypeptidechains are represented by thick lines;disulfide bonds linking differentpolypeptide chains are representedby thin lines. (Reproduced, with permission,from Parslow TG et al [editors]:Medical Immunology, 10th ed.McGraw-Hill, 2001.)C. lgM(pentamer)HLHLJ chainspecific immunoglobulin molecules, the latter by clonaltumors of plasma cells called myelomas. Multiplemyeloma is a neoplastic condition; electrophoresis ofserum or urine will usually reveal a large increase of oneparticular immunoglobulin or one particular light chain(the latter termed a Bence Jones protein). Decreasedproduction may be restricted to a single class of immunoglobulinmolecules (eg, IgA or IgG) or may involveunderproduction of all classes of immunoglobulins(IgA, IgD, IgE, IgG, and IgM). A severe reductionin synthesis of an immunoglobulin class due to a geneticabnormality can result in a serious immunodeficiencydisease—eg, agammaglobulinemia, in whichproduction of IgG is markedly affected—because ofimpairment of the body’s defense against microorganisms.Hybridomas Provide Long-Term Sourcesof Highly Useful Monoclonal AntibodiesWhen an antigen is injected into an animal, the resultingantibodies are polyclonal, being synthesized by amixture of B cells. Polyclonal antibodies are directedagainst a number of different sites (epitopes or determinants)on the antigen and thus are not monospecific.However, by means of a method developed by Kohlerand Milstein, large amounts of a single monoclonal antibodyspecific for one epitope can be obtained.The method involves cell fusion, and the resultingpermanent cell line is called a hybridoma. Typically, Bcells are obtained from the spleen of a mouse (or othersuitable animal) previously injected with an antigen ormixture of antigens (eg, foreign cells). The B cells are