Page 243 - Clinical Immunology_ Principles and Practice ( PDFDrive )

224 Part two Host Defense Mechanisms and Inflammation

The intrinsic affinity of an antibody for a small molecule, than the intrinsic affinity of that antibody for the relevant epitope
such as a drug (e.g., digoxin) or a hormone (e.g., insulin), can on that same antigen. 2
3
be clinically important both in vivo and in vitro. For example, Functional affinity is also influenced by the degree to which
the in vivo effectiveness of antibody F(ab) fragments in removing the geometrical relationships among the epitopes are optimal
toxic levels of the drug, digoxin, from patients being treated for for the paratopes, which will depend on the quaternary structure
congestive heart failure likely depends on the intrinsic affinity and segmental flexibility of the antibody molecule. In the presence
of the F(ab) fragments for the drug. Alternatively, antibody of nonoptimal geometry, the average number of engaged sites
intrinsic affinity can limit the analytical sensitivity of an in vitro may be less than maximal, and energy may be expended in
immunoassay designed to determine the concentration of an achieving some epitope–paratope contacts. Therefore the func-
analyte, such as a hormone (e.g., insulin, parathyroid hormone) tional affinity for a multivalent interaction does not necessarily
or a drug (e.g., digoxin). increase in direct relationship to the maximal number of binding
In contrast, functional affinity is defined as the equilibrium sites that can be engaged simultaneously by an antibody molecule.
association constant characterizing the interaction between an For example, the effective valency of pentameric IgM with 10
intact antibody and an intact antigen. For a monovalent immu- paratopes is typically half that.
noglobulin G (IgG) antibody–antigen interaction, the intrinsic Intrinsic affinity provides information, qualified as above, on
affinity and the functional affinity will be the same. However, if the degree of complementarity between the epitope and the
two paratopes interact simultaneously with two epitopes on the paratope. Functional affinity accounts for properties influenced
same antigen, referred to as monogamous bivalency (Fig. 15.2), by structural features outside of the epitope and the paratope,
the functional affinity of the antibody for the multivalent antigen as normally conceived. Both concepts of affinity are valuable.
may be substantially greater (as much as 10 000-fold for IgG) Maximization of intrinsic affinity may be of prime importance
for antibody-mediated inactivation of toxins or enzymes, which
frequently involve monovalent interactions. However, in cases
where antibodies bind to repeated epitopes on the surfaces of
TABLE 15.1 antigens and Valence bacteria, viruses, fungi, parasites, or mammalian cells, the
functional affinity may play a much larger role influencing the
Number of types Epitope Copy biological consequences of the interaction.
of Epitope Number Examples Bivalent (IgG, IgE) or multivalent (IgA, IgM) antibodies carry
Monodeterminant Monovalent Hapten: DNP, digoxin with them the potential to bind simultaneously to two or more
Monodeterminant Multivalent Polysaccharide: dextran a epitopes on different antigenic particles, cross-linking them rather
Multideterminant b Monovalent Monomeric protein: than engaging in monogamous bivalency or monogamous
myoglobin
Multideterminant Multivalent Virion: influenza virus multivalency (see Fig. 15.2). This phenomenon has played an
important historical role in immunology. It is the basis for the
a Even a polysaccharide composed of one type of hexasaccharide can have two or clinical method for typing erythrocyte antigens (e.g., ABO and
more different kinds of epitope: terminal versus internal residues, for instance. Rh antigens), which still rely on agglutination of red cells by
However, a given antiserum may preferentially contain antibodies specific for only
one such epitope. antibodies (or lectins). Such antibodies may also participate in
b Typically, multideterminant recognition occurs with respect to a polyclonal antibody. the inactivation of complex antigens such as are found on the
Adapted from Benjamini E, Leskowitz S. Immunology: a short course, 2nd ed.
New York: Wiley-Liss; 1991, with permission from Wiley-Liss, Inc., a subsidiary of surface of human immunodeficiency virus (HIV). Many neutral-
John Wiley & Sons, Inc. izing epitopes of HIV are normally not well exposed on native

Antigen alone Antigen + antibody

Dialysis Free antibody
membrane

Antigen
molecules
Antibody with
bound antigen

A B
FIG 15.1 Measurement of the Intrinsic Affinity Characterizing an Interaction Between Antibody
and Antigen (Hapten) by Equilibrium Dialysis. At equilibrium (A), the amount of diffusible free
hapten inside the dialysis bag will be equal to the amount of free hapten outside of the dialysis
bag. However, in the presence of hapten-specific antibody (B), the total hapten concentration
will be greater inside of the dialysis bag (free + antibody-bound) than outside of the bag (free).
The extent of this difference can be used to determine the intrinsic affinity of the antibody for
the hapten. With permission from Abbas AK, Lichtman AH, Pober JS. Cellular and molecular
immunology. W. B. Saunders Company; 1991.