Antign-Antibodyng 131.the ceiling originates from the limits for association and Biological Significance of Antibody dissociation rate constants.The maximum association rate Affinity and Multivalency constant for verified experimentally.In contrast.no limitation for nihheaiaiyorsantienano时 n the i dissociation rate constant scems to exist,and affinity reactions such as haemagglutination,virus neutralization, changes appear mostly as variations of the dis iation rate le is fixed at considered that dissociation rates which are too slow would the primary naive antibody library.and not be selected in the immune system.Thus,the affinity usually increases during an immune response (called antibody for any antigen is around re provides us with c af hle of h the low intrinsic affinity of IgM produced during the Affinity and Avidity primary immune response is compensated b its penta merc structure, resulting in a high avidity towar The str of bacteriaor viruses.Thus.one of the most efficient ways attractive and repulsive fore to increase the binding activity of an antibody to a surface es mentioned al ove.However e multivalency eftec ince cac onal ced by hyb elsurface)is to makeus two antigen nity of an ant which can bind multiple antigenic determinants,antibodies are potentially multivalent in their reaction with antigen.When affinity,leading to the production of antibody molecules an antigen carrying multiple copies of the greatly increased becauseall ofthe antigen-antibody bonds must be broken simulta before the antigen and Further Reading 6 sites of an ody is than th Arevalo JH.Taussi MI and Wils of the affinity of each binding site for an antigen. erm ntiat Bhat TN.Mariuzza RA.Poljak RJeral.(1994)Bound water molecules t an 093. of an antibody for its antigen is determined by the sum of D etwe Chitarra V.Alzari PM.Poljak RJeral.(1993)Three e-dime al structure antigen strongly depends on the of the individua combining sites for the determinants on the antigens.It is of the US: Chothia C Tramo tano A er al.(19 ations of greater thar the summation of these affinities if both antgen-bindi sof anan ntigens with (IgG)up to 10 (IgM).and the advanta ofmultivalence te 0S4937-12 the functional affinity (as opposed to the affinity of Davies EA and termed ()MOLSCRIPT.Jo fpplied 24: mate n e in immune respon in the immune system. HyHEL-S.: 6 ENCYCLOPEDIA OF LIFE SCIENCES/2001 Na ww.els.net[3], the ceiling originates from the limits for association and dissociation rate constants. The maximum association rate constant for the binding of a monomeric protein antigen by its antibody is approximately 105 –106 , a value controlled by the diffusion coefficients of the reactant molecules and verified experimentally. In contrast, no limitation for dissociation rate constant seems to exist, and affinity changes appear mostly as variations of the dissociation rate constant. Nevertheless, the kdiss of the naturally prepared antibody molecule is fixed at 10 2 3 –10 2 4 , and it is considered that dissociation rates which are too slow would not be selected in the immune system. Thus, the affinity ceiling of an antibody for any antigen is around 1010L mol 2 1 . Affinity and Avidity The strength of a single antigen–antibody bond is termed the antibody affinity. It is produced by summation of the attractive and repulsive forces mentioned above. However, since each monoclonal antibody produced by hybridoma technology has two antigen-binding sites and antibodies obtained from serum contain polyclonal antibodies which can bind multiple antigenic determinants, antibodies are potentially multivalent in their reaction with antigen. When an antigen carrying multiple copies of the antigenic determinant (macromolecules or microorganisms) com￾bines with a multivalent antibody, the binding strength is greatly increased because all of the antigen–antibody bonds must be broken simultaneously before the antigen and antibody can dissociate. Thus, total binding energy between a multivalent antigen and more than one of the antigen￾binding sites of an antibody is greater than the summation of the affinity of each binding site for an antigen. The strength with which a multivalent antibody binds a multivalent antigen is termed avidity, to differentiate it from the affinity of the bond between a single antigenic determinant and an individual combining site. The avidity of an antibody for its antigen is determined by the sum of all of the individual interactions taking place between individual antigen-binding sites of antibodies and deter￾minants on the antigens. The avidity of an antibody for its antigen strongly depends on the affinities of the individual combining sites for the determinants on the antigens. It is greater than the summation of these affinities if both antigen-binding sites of an antibody can combine with the antigen. The effective range of antibody valence is from 2 (IgG) up to 10 (IgM), and the advantage of multivalence to the functional affinity (as opposed to the affinity of monovalent interactions, termed intrinsic affinity) is estimated to be 103–7. Thus, even when each antigen￾binding site has only a lowaffinity (e.g. IgM produced early in immune responses), antibodies can function effectively in the immune system. Biological Significance of Antibody Affinity and Multivalency An antibody with high affinity for its antigen can function most effectively in the immune system (e.g. in biological reactions such as haemagglutination, virus neutralization, enzyme inactivation, haemolysis, immune elimination of foreign antigens, etc.). However, an antibody molecule with high affinity for target antigen does not usually exist in the primary naive antibody library, and antibody affinity usually increases during an immune response (called affinity maturation) in vivo. Nature provides us with numerous examples of molecules with low-intrinsic affinity binding sites that are capable of high-avidity interactions with their targets due to multivalent binding. For example, the lowintrinsic affinity of IgM produced during the primary immune response is compensated by its penta￾meric structure, resulting in a high avidity toward repetitive antigenic determinants present on the surface of bacteria or viruses. Thus, one of the most efficient ways to increase the binding activity of an antibody to a surface (e.g. cell surface) is to make use of the multivalency effect. Therefore, even if the intrinsic affinity of an antibody molecule toward various invaders (e.g. viruses, proteins) is relatively low, high avidity can overcome the low intrinsic affinity, leading to the production of antibody molecules with high intrinsic affinity for the target antigen through affinity maturation in the immune system. Further Reading Arevalo JH, Taussing MJ and Wilson IA (1993) Molecular basis of crossreactivity and the limits of antibody–antigen complementarity. Nature 365: 859–863. Bhat TN, Mariuzza RA, Poljak RJ et al. (1994) Bound water molecules and conformational stabilization help mediate an antigen–antibody association. Proceedings of the National Academy of Sciences of the USA 91: 1089–1093. Branden C and Tooze J (eds) (1991) Recognition of foreign molecules by the immune system. Introduction to Protein Structure, chap. 12. New York: Garland Publishing. Chitarra V, Alzari PM, Poljak RJ et al. (1993) Three-dimensional structure of a heteroclitic antigen–antibody cross-reaction complex. Proceedings of the National Academy of Sciences of the USA 90: 7711–7715. Chothia C, Lesk AM, Tramontano A et al. (1989) Conformations of immunoglobulin hypervariable regions. Nature 342: 877–883. Davies DR and Cohen GH (1996) Interactions of protein antigens with antibodies. Proceedings of the National Academy of Sciences of the USA 93: 7–12. Davies DR, Padlan EA and Sheriff S (1990) Antibody–antigen complexes. Annual Reviewof Biochemistry 59: 439–473. Kraulis PJ (1991) MOLSCRIPT. Journal of Applied Crystallography 24: 946–950. Novotny J, Bruccoleri RE and Saul FA (1989) On the attribution of binding energy in antigen–antibody complexes McPC603, D1.3 and HyHEL-5. Biochemistry 28: 4735–4749. Antigen–Antibody Binding 6 ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net