Molecular Basis of Antigen - Antibody Reactions
المؤلف:
Mary Louise Turgeon
المصدر:
Immunology & Serology in Laboratory Medicine
الجزء والصفحة:
5th E, P22-23
2025-05-19
658
The basic Y-shaped Ig molecule is a bifunctional structure. The V regions are primarily concerned with antigen binding. When an antigenic determinant and its specific antibody combine, they interact through the chemical groups found on the surface of the antigenic determinant and on the surface of the hypervariable regions of the Ig molecule. Although the C regions do not form antigen-binding sites, the arrangement of the C regions and hinge region give the molecule segmental flexibility, which allows it to combine with separated antigenic determinants.
Types of Bonding
Bonding of an antigen to an antibody result from the formation of multiple, reversible, intermolecular attractions between an antigen and amino acids of the binding site. These forces require proximity of the interacting groups. The optimum distance separating the interacting groups varies for different types of bond; however, all these bonds act only across a very short distance and weaken rapidly as that distance increases.
The bonding of antigen to antibody is exclusively noncovalent. The attractive force of noncovalent bonds is weak compared with that of covalent bonds, but the formation of multiple noncovalent bonds produces considerable total binding energy. The strength of a single antigen-antibody bond (antibody affinity) is produced by the summation of the attractive and repulsive forces. The four types of noncovalent bonds involved in antigen-antibody reactions are hydrophobic bonds, hydro gen bonds, van der Waals forces, and electrostatic forces.
Hydrophobic Bonds
The major bonds formed between antigens and antibodies are hydrophobic. Many of the nonpolar side chains of proteins are hydrophobic. When antigen and antibody molecules come together, these side chains interact and exclude water molecules from the area of the interaction. The exclusion of water frees some of the constraints imposed by the proteins, which results in a gain in energy and forms an energetically stable complex.
Hydrogen Bonds
Hydrogen bonding results from the formation of hydrogen bridges between appropriate atoms. Major hydrogen bonds in antigen-antibody interactions are O–H–O, N–H–N, and O–H–N.
Van der Waals Forces
Van der Waals forces are nonspecific attractive forces generated by the interaction between electron clouds and hydrophobic bonds. These bonds result from minor asymmetry in the charge of an atom caused by the position of its electrons. They rely on the association of nonpolar hydrophobic groups so that contact with water molecules is minimized. Although extremely weak, van der Waals forces may become collectively important in an antigen-antibody reaction.
Electrostatic Forces
Electrostatic forces result from the attraction of oppositely charged amino acids located on the side chains of two amino acid residues. The relative importance of electrostatic bonds is unclear.
Goodness of Fit
The strongest bonding develops when antigens and antibodies are close to each other and when the shapes of the antigenic determinants and the antigen-binding site conform to each other. This complementary matching of determinants and binding sites is referred to as goodness of fit (Fig. 1).
Fig1. Goodness of fit.
A good fit will create ample opportunities for the simultaneous formation of several noncovalent bonds and few opportunities for disruption of the bond. If a poor fit exists, repulsive forces can overpower any small forces of attraction. Variations from the ideal complementary shape will produce a decrease in the total binding energy because of increased repulsive forces and decreased attractive forces. Goodness of fit is important in determining the binding of an antibody molecule for a particular antigen.
Detection of Antigen-Antibody Reactions
In vitro tests detect the combination of antigens and antibodies. Agglutination is the process whereby particulate antigens (e.g., cells) aggregate to form larger complexes in the presence of a specific antibody. Agglutination tests are widely used in immunology to detect and measure the consequences of antigen antibody interaction. Other tests include the following:
• Precipitation reactions combine soluble antigen with soluble antibody to produce insoluble complexes that are visible.
• Hemolysis testing involves the reaction of antigen and antibody with a cellular indicator (e.g., lysed RBCs).
• The enzyme-linked immunosorbent assay (ELISA) measures immune complexes formed in an in vitro system.
The principles of immunologic methods are discussed in Part II of this text. Detection and quantitation of immunoglobulins is important in the laboratory investigation of infectious diseases and immunologic disorders (Table 1).
Table1. Role of Specific Immunoglobulins in Diagnostic Tests
Influence of Antibody Types on Agglutination
Immunoglobulins are relatively positively charged and, after sensitization or coating of particles, they reduce the zeta potential, which is the difference in electrostatic potential between the net charge at the cell membrane and the charge at the surface of shear . Antibodies can bridge charged particles by extending beyond the effective range of the zeta potential, which results in the erythrocytes closely approaching each other, binding, and agglutinating.
Antibodies differ in their ability to agglutinate. IgM-type antibodies, sometimes referred to as complete antibodies, are more efficient than IgG or IgA antibodies in exhibiting in vitro agglutination when the antigen-bearing erythrocytes are sus pended in physiologic saline (0.9% sodium chloride solution). Antibodies that do not exhibit visible agglutination of saline suspended erythrocytes, even when bound to the cell’s surface membrane, are considered to be nonagglutinating antibodies and have been called incomplete antibodies. Incomplete anti bodies may fail to exhibit agglutination because the antigenic determinants are located deep within the surface membrane or may show restricted movement in their hinge region, causing them to be functionally monovalent.
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