Section I. INTRODUCTION
Since 1970, immunological laboratory methods have gradually become
increasingly more refined and simplified. Because of their improved specificity and
sensitivity, these methods have now achieved a major role in modern clinical laboratory
science. As many new laboratory tests employing immunologic principles are
developed, these methods of laboratory diagnosis have often been applied to clinical
You will be better able to interpret and apply the new knowledge about
immunology as you develop an understanding of the methods now in use. In this
lesson, the techniques and applications of the various immunological tests for the
detection of antigens and antibodies are discussed.
a. In 1934 Marrack, a prominent Englishman, proposed a new model for
antigen-antibody reactions. His hypothesis states that under appropriate experimental
conditions, antigen-antibody complexes precipitate. Antibody molecules are bivalent,
that is, they contain two antigen-binding sites. For this reason, when antibodies are
complexed with antigen, they can form a cross-linked mass. At a proper antigen and
antibody concentration, this cross-linked mass enlarges and precipitates.
b. At the onset of the antigen-antibody reaction, an invisible formation of antigen
and antibody complexes occurs. A lattice of soluble complexes slowly develops and
gradually expands into a visible precipitate (lattice formation) as the antibody and
antigen reach a zone of equivalent concentration. If there is an excess of either antigen
(postzone) or antibody (prozone) present, the proper lattice formation needed for
precipitate formation cannot occur. The formation of the antigen-antibody complex is
reversible and may dissolve if more antigen or antibody is added.