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chapter

35

Molecular Immunology

Monomer

IgG, IgE, IgD

Dimer

IgA

Secretory

“ component

Pentamer

IgM

J chain

F I G U R E 3 5 -1 1

(Also see color figure; color coding is as the same as indicated in Fig. 35-7.) Immunoglobulin classes. The five

immunoglobulin classes are determined by the amino acid sequences of the constant regions of the heavy

polypeptide chains (see Table 35-2).

Antigen-Antibody Reactions

Under normal circumstaces an animal produces a mix-

ture of antibodies because of there being several epitopes

of an antigen. This is because antibodies that react with

the different epitopes of the same antigen are produced

by many different B-cell clones (see “B-Cell Clonal Se-

lection and Proliferation”). Each B-cell clone synthesizes

antibodies to a single epitope and thus the total antibody

that is specific for the antigen will be a mixture of single-

clone-derived antibodies. The total antibody is there-

fore polyclonal with the antibody molecules from each

clone recognizing different epitopes on the same antigen

molecule.

The two Fab “arms” of the reference immunoglobulin

molecule, each with the ability to recognise antigen, make

the antibody molecule bivalent or divalent. The bivalent

immunoglobulin molecules are IgG, IgA, IgE, IgD. IgM

molecules, as described above, are decavalent and can

react with as many as 10 antigen molecules. One form of

IgA can bind four antigen molecules. The consequence

of bivalency and multivalency is that antibodies can

react with antigens to form aggregates composed of

alternating Ag-Ab-Ag-Ab molecules. This cross-linking

results in the formation of large Ag-Ab aggregates.

Such cross-linking is strictly true only for polyclonal

antibodies, i.e., mixtures of monoclonal antibodies that

contain antibodies to more than a single epitope on the

antigen. If an antigen molecule possesses two copies

of the epitope that the monoclonal Ab recognizes, then

monoclonal antibodies can cross-link also.

The Ag-Ab aggregates may become sufficiently large so

that solutions become visibly turbid. The complexes can

precipitate from the solution. This phenomenon is called

immunoprécipitation

and is the basis of many antibody-

based diagnostic tests. It is used to deplete plasma samples

of a particular protein. Immuno-absorbed plasmas find

widespread use in tests of specific coagulation factors.

The formation of aggregates of red blood cells that oc-

curs when cells of one blood type are mixed with plasma

from an individual of a different blood type is another

example of Ab-Ab cross-linking. Red blood cell aggrega-

tion and precipitation is the basis for blood typing and

cross-matching procedures of immunohematology. The

formation of bands in gels using immunoelectrophoresis

or immunodiffusion procedures involves laboratory meth-

ods that exploit the property of bivalency (or multivalency)

of immunoglobulins.

Because of the bi- and multivalency of antibodies, the

formation of antigen-antibody aggregates is sensitive to

the

relative

concentrations of antigen and antibody. Op-

timal precipitation occurs at a concentration called the

equivalence point. At too high an antigen concentration,

the binding of antigen molecules to antibody is saturated

and the aggregates become simple complexes between

each antibody and the two antigens. At too high an an-

tibody concentration, all of the antibody binding sites

on the antigen are occupied by individual antibodies and

sites are not available to form cross-links. The complexes

that contain either excess antigen or excess antibody are

usually soluble and thus do not lead to identification of