Antibody Avidity Incorporates Affinity of Multiple Binding Sites

The affinity at one binding site does not always reflect the true strength of the antibody-antigen interaction. When complex antigens containing multiple, repeating antigenic determinants are mixed with antibodies containing multiple binding sites, the interaction of an antibody molecule with an antigen molecule at one site will increase the probability of reaction between those two molecules at a second site. The strength of such multiple interactions between a multivalent antibody and antigen is called the avidity. The avidity of an antibody is a better measure of its binding capacity within biological systems (e.g., the reaction of an antibody with anti-genic determinants on a virus or bacterial cell) than the affinity of its individual binding sites. High avidity can compensate for low affinity. For example, secreted pentameric

(a) Homogeneous antibody #1

(a) Homogeneous antibody #1

Intercept = n

Homogeneous Antibody

Intercept = n

(b) Heterogeneous antibody

#3

#4

- \

, Slope at r of 1/2 n = -K0

£-----

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^Intercept =

FIGURE 6-3

Scatchard plots are based on repeated equilibrium dialyses with a constant concentration of antibody and varying concentration of ligand. In these plots, r equals moles of bound lig-and/mole antibody and c is the concentration of free ligand. From a Scatchard plot, both the equilibrium constant (Ka) and the number of binding sites per antibody molecule (n), or its valency, can be obtained. (a) If all antibodies have the same affinity, then a Scatchard plot yields a straight line with a slope of — Ka. The x intercept is n, the valency of the antibody, which is 2 for IgG and other divalent Igs. For IgM, which is pentameric, n = 10, and for dimeric IgA, n = 4. In this graph, antibody #1 has a higher affinity than antibody #2. (b) If the antibody preparation is polyclonal and has a range of affinities, a Scatchard plot yields a curved line whose slope is constantly changing. The average affinity constant K0 can be calculated by determining the value of Ka when half of the binding sites are occupied (i.e., when r = 1 in this example). In this graph, antiserum #3 has a higher affinity (K0 = 2.4 X 108) than antiserum #4 (K0 = 1.25 X 108). Note that the curves shown in (a) and (b) are for divalent antibodies such as IgG.

IgM often has a lower affinity than IgG, but the high avidity of IgM, resulting from its higher valence, enables it to bind antigen effectively.

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Responses

  • andreas
    How to measure antigen antibody avidity?
    8 years ago
  • girma
    How avidity compensate affinity?
    3 years ago

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