SOLUTIONS TO EXERCISES, LESSON 5
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.
(paras 5-1 thru 5-2)
In Marrack's model, under appropriate experimental conditions, antigen-antibody
complexes precipitate. Antibody molecules are bivalent, that is, they contain two
antigen-binding sites. At a proper antigen and antibody concentration, a cross-
linked mass enlarges and precipitates.
At the onset of the antigen-antibody reaction, an invisible formation of antigen and
antibody complexes occurs. This is followed by an expanding visible precipitate,
called a lattice formation, as the antigen and antibody reach a zone of equivalent
concentration. If there is an excess of either antigen or antibody 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.
Immunodiffusion techniques detect antigen-antibody precipitation reactions in a
semisolid medium. The formation of antigen and antibody complexes can be
influenced by a number of factors: relative concentration of antigens and
antibodies, ionic strength of the buffer, pH, and temperature. Two techniques
most often used in a clinical laboratory are single and double diffusion.
In the r_____l immunodiffusion (RID) method, a known concentration of antibody
is incorporated into an agarose medium. The reactant (antigen) is applied to a
well cut in the agarose and radially diffuses from the site of application. At the
point of equivalence, the antigen and antibody react to form a visible precipitin
ring. The size of the precipitin ring is proportional to the concentration of the
antigen. In the clinical laboratory, radial immunodiffusion is primarily used to
quantitate serum immunoglobulins and complement components such as C3 or