Immunoassay techniques
are used for nanogram level determination of hormones, drugs and vitamins. This
method involves formation of a complex precipitate as a result of reaction of
antigen and antibody. For measuring small amounts of insulin, this was first
employed and the method was known as radioimmunoassay. The complex precipitate
that formed as a result of antigen-antibody reaction is known as
antigen-antibody complex. The intrinsic association constant between an
antibody and a univalent antigen is known as affinity. The total binding energy
between antibodies and a multivalent antigen is known as avidity. The three
parameters which influence the specificity of immunoassays are heterogeneity of
the antibody, cross reaction with other antigens and possible interferences of
the antigen-antibody reaction from low-molecular weight substances which is
altering the environment of the reaction. Use of monoclonal antibodies enhances
the specificity. Two popular immunoassay techniques are enzyme-linked
immunosorbent assays and fluorescence immunoassays.
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